fix(agent-tools): honor falsey custom output extractors

[n33levo]

Summary

Agent.as_tool(custom_output_extractor=...) currently checks the configured extractor by truthiness. A callable object whose __bool__ returns False is accepted by the public API but silently skipped, so the nested agent returns its default output instead of the custom result.

This changes the guard to an explicit is not None check. None keeps the existing fallback behavior; ordinary async functions and the public API are unchanged.

Test plan

• env UV_DEFAULT_INDEX=https://pypi.org/simple bash .agents/skills/code-change-verification/scripts/run.sh - format, lint, typecheck, and full tests passed
• make coverage - 6,034 passed, 8 skipped; 91% total coverage
• make tests-asyncio-stability - 5 repetitions passed (7 + 33 tests per repetition)
• Python 3.10 focused tests - 2 passed
• Python 3.14 focused tests - 2 passed

E2E top-hat using the public Agent.as_tool invocation path:

• v0.19.2 returned default-output and skipped the falsey extractor
• This branch returned custom-output and invoked the extractor once

Issue number

N/A - searches across open and closed issues and pull requests found no duplicate.

Checks

• I've added new tests, if relevant
• I've run .agents/skills/code-change-verification/scripts/run.sh
• I've confirmed all verification steps pass