fix: replace assert with runtime guards in computer_use (browser_route, doctor)

[JonthanaHanh]

Summary

Replace assert statements with explicit runtime guards in tools/computer_use/ — two sites not covered by prior assert-cleanup PRs (#56736, #56866, #61983, #62001, #62659, #64818).

assert is stripped by python -O, silently removing invariant checks in production.

Changes

File	Line	Old	New
tools/computer_use/browser_route.py	481	assert selected_tab is not None and self.state.target_id is not None	if ... is None: return _refusal(...)
tools/computer_use/doctor.py	226	assert proc.stdin is not None and proc.stdout is not None	if ... is None: raise RuntimeError(...)

Why these matter

• browser_route.py: After _require_mutation() returns with no refusal, subsequent code dereferences selected_tab and self.state.target_id. Under python -O, if either is None, the code crashes with AttributeError deep in the browser tool dispatch — a confusing error with no context. The guard returns a clear refusal dict.

• doctor.py: _mcp_rpc() writes to proc.stdin and reads from proc.stdout. If either is None (e.g. subprocess started with stdin=DEVNULL), the code crashes with AttributeError: 'NoneType' object has no attribute 'write'. The guard raises RuntimeError with a descriptive message.

Test Plan

• python3 -c "import ast; ast.parse(open('tools/computer_use/browser_route.py').read())" — syntax OK
• python3 -c "import ast; ast.parse(open('tools/computer_use/doctor.py').read())" — syntax OK
• Both files pass ruff check