[arnav] remove dead code identified by audit

Each removal verified by checking actual production callers (frontend,
electron, internal HTTP, MCP-server subprocesses) — not just test
references. Symbols whose only callers were tests are removed along
with those tests.

Production removals (~390 LOC):

- backend/main.py
  - websocket_session: drop `agent:edit_message` WS branch. Frontend
    only ever uses HTTP `POST /api/agents/sessions/{id}/edit_message`
    (frontend/src/shared/state/agentsSlice.ts); nothing on the wire
    sends a WS message of this type.

- backend/apps/agents/agent_manager.py
  - AgentManager._build_connected_tools_context (~80 LOC): zero call
    sites in production; the connected-tools system-prompt context is
    built inline in _compose_system_prompt now.
  - AgentManager._approx_tokens / _summarize_message_block: pure
    helpers whose only callers were tests. The compaction path uses
    LLM-driven _maybe_compact instead.

- backend/apps/agents/browser_agent.py
  - clear_browser_history: only used by tests. _browser_history is
    pruned via the size cap inline.
  - MODEL_MAP constant: never read.

- backend/apps/agents/mcp_preflight.py
  - DISCOVERY_SCAFFOLDING (~25-line system-prompt block): defined but
    never appended anywhere. The header comment described an intended
    use that the codebase no longer has.

- backend/apps/agents/providers/registry.py
  - thinking_params_for, _is_9router_available, OPENROUTER_BASE_URL,
    get_context_window: zero callers in production. Thinking-params
    routing is done by the provider classes directly; 9Router presence
    is detected at request time; context-window numbers are stamped
    onto sessions from BUILTIN_MODELS at launch.

- backend/apps/agents/tools/{base,web}.py
  - BaseTool.get_schema (abstract) + WebSearchTool/WebFetchTool
    overrides: production code in backend/apps/web/web.py instantiates
    these tools and only calls .execute(); the JSON-schema lives in
    the HTTP wrapper, not on the tool class.

- backend/apps/outputs/outputs.py
  - _resolve_model + MODEL_MAP: tests-only.
  - load_output: docstring claimed it was a public helper for "other
    modules" but no module imported it.

- backend/apps/service/client.py
  - set_user_id, the _user_id module global, and the dead cache short-
    circuit in _get_user_id: setter was tests-only. _get_user_id now
    reads user_email directly from settings on every call.

- backend/apps/settings/credentials.py
  - get_provider_credentials: zero callers. The sibling get_agent_sdk_env
    is kept (it has the explicit "Legacy helpers" keep-comment).

Test updates:

- test_agent_manager_unit.py: drop _approx_tokens / _summarize_message_block
  cases (5 tests), update module docstring index.
- test_browser_agent_unit.py: drop clear_browser_history cases (2 tests)
  and the unused _Boom helper class in the repr-fallback test.
- test_outputs_unit.py: drop _resolve_model / load_output cases
  (4 tests), update docstring + import list.
- test_v2_invariants.py: drop get_context_window tests + get_schema
  assertions on web tools (kept name + BaseTool inheritance checks).
- test_service.py: rewrite the 4 set_user_id-driven tests to drive
  user_id through settings.user_email instead, so _get_user_id's live
  envelope-stamping path stays covered.

Verification:
- ruff --select F401,F811,F841 backend/  →  clean.
- pytest backend/tests/ → 1167 passed, 1 deselected (pre-existing
  sandbox git test, unrelated). No tests dropped silently — every
  deletion is paired with the corresponding test removal/rewrite.
- Dead-code scan re-run: dead WS events 1→0, Tier-2 high-confidence
  14→11 (residue is SDK-callback `context` params + Pydantic `cls`
  validators — both false positives vulture can't see through),
  vulture total 165→145.

Total diff: -565 / +34 LOC across 15 files.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_agent_manager_unit.py

@@ -12,8 +12,7 @@ Test groups:
   - pure instance methods (`_resolve_mode`, `_compose_system_prompt`,
     `_resolve_context_paths`, `_build_dir_tree`, `_resolve_forced_tools`,
     `_resolve_attached_skills`, `_get_branch_messages`,
-    `_build_history_prefix`, `_approx_tokens`,
-    `_summarize_message_block`, `_truncate_large_tool_result`,
+    `_build_history_prefix`, `_truncate_large_tool_result`,
     `_build_search_text`, `_maybe_compact`)
   - lifecycle (launch/update/edit/switch_branch/duplicate/close/
     delete/resume/stop, history, browser-agent children, approval,
@@ -564,7 +563,7 @@ def test_get_branch_messages_walks_fork_lineage():
 
 
 # ---------------------------------------------------------------------------
-# _build_history_prefix / _approx_tokens / _summarize_message_block
+# _build_history_prefix
 # ---------------------------------------------------------------------------
 
 
@@ -587,59 +586,6 @@ def test_build_history_prefix_empty_returns_empty():
     assert AgentManager._build_history_prefix([]) == ""
 
 
-@pytest.mark.parametrize("text,expected", [
-    ("", 1),
-    ("a" * 4, 1),
-    ("a" * 16, 4),
-    ("a" * 100, 25),
-])
-def test_approx_tokens_chars_over_four(text, expected):
-    assert AgentManager._approx_tokens(text) == expected
-
-
-def test_approx_tokens_handles_none():
-    assert AgentManager._approx_tokens(None) == 1
-
-
-def test_summarize_message_block_empty_returns_empty():
-    assert AgentManager._summarize_message_block([]) == ""
-
-
-def test_summarize_message_block_extracts_initial_task_and_counts():
-    msgs = [
-        Message(role="user", content="please do the thing"),
-        Message(role="tool_call", content={"tool": "Read", "input": {}}),
-        Message(role="tool_call", content={"tool": "Bash", "input": {}}),
-        Message(role="tool_call", content={"tool": "Read", "input": {}}),
-        Message(role="tool_result", content="ok"),
-        Message(role="assistant", content="done"),
-    ]
-    out = AgentManager._summarize_message_block(msgs)
-    assert "<compacted_history>" in out
-    assert "please do the thing" in out
-    # Tool counts
-    assert "Read×2" in out
-    assert "Bash×1" in out
-    assert "Tool calls so far (3 total)" in out
-    assert "Tool results received: 1" in out
-    assert "Last assistant message:" in out
-    assert "done" in out
-
-
-def test_summarize_message_block_assistant_list_content():
-    """Assistant messages with list content (Anthropic block shape)
-    should still surface their text."""
-    msgs = [
-        Message(role="user", content="task"),
-        Message(role="assistant", content=[
-            {"type": "text", "text": "the answer"},
-            {"type": "tool_use", "id": "t1"},
-        ]),
-    ]
-    out = AgentManager._summarize_message_block(msgs)
-    assert "the answer" in out
-
-
 # ---------------------------------------------------------------------------
 # _truncate_large_tool_result
 # ---------------------------------------------------------------------------