fix: #3330 handle string tool trimmer allowlists

[Aphroq]

Summary

• Treat a string ToolOutputTrimmer.trimmable_tools value as one tool name instead of iterating it into characters.
• Keep list, set, and frozenset allowlists normalized to frozenset, and reject bytes so byte strings do not become integer allowlists.
• Add regression coverage for single-string allowlists matching the intended tool name without matching single-character tool names.

Test plan

• uv run pytest tests/extensions/test_tool_output_trimmer.py -k "trimmable_tools or allowlist"
• uv run mypy src/agents/extensions/tool_output_trimmer.py tests/extensions/test_tool_output_trimmer.py
• uv run pytest tests/extensions/test_tool_output_trimmer.py
• bash .agents/skills/code-change-verification/scripts/run.sh

Issue number

Closes #3330

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass