feat(langchain): detect Requests calls without timeout

[pjsk02]

Summary

Adds LC-007 to detect Python requests calls without a finite timeout inside LangChain tools.

LangChain Python currently lacks equivalent timeout coverage while similar network-timeout rules exist for other supported SDKs. This adds that coverage using Trustabl''s existing call_without_kwarg predicate without requiring engine or schema changes.

Detection

LC-007 fires for supported requests calls inside LangChain tools when:

• timeout is omitted
• timeout=None

It remains silent when an explicit finite timeout is supplied.

The rule intentionally does not flag httpx merely because a call omits an explicit timeout, since HTTPX provides default timeout behavior.

Scope

This contribution is intentionally limited to Python Requests call-site detection.

Known limitations include wrapper functions, dynamically configured client defaults, positional timeout handling, and HTTP libraries outside the current Requests-specific scope.

Validation

• Existing call_without_kwarg predicate reused
• No production engine changes
• No schema changes
• No manifest changes
• Canonical rule mirrored byte-for-byte in the companion engine fixture
• git diff --check passes

Companion PRs

• Engine fixture/tests: test(langchain): cover missing network timeout rule trustabl#191
• Rulebook rationale: docs(langchain): document network timeout policy trustabl-rulebook#107