feat: add stream_validate() hook to Requirement (#900)

[planetf1]

⚠️ Stacked PR — read before reviewing

This is a stacked PR (Wave 2 of streaming epic #891). The diff against main includes code from the two Wave 1 PRs that this branch builds on:

PR	Contents	Status
#924	PartialValidationResult tri-state type	Ready to merge
#923	ChunkingStrategy ABC + built-in chunkers	Ready to merge
#925 (this PR)	stream_validate() hook on Requirement	Draft

Suggested review order:

1. Review and approve feat(core): add PartialValidationResult with tri-state semantics #924 first — it is self-contained and has no dependencies
2. Review and approve feat(stdlib): add ChunkingStrategy ABC and built-in chunkers #923 — also self-contained
3. Return here and focus only on the two files new to this PR (below)

What to ignore in the diff: mellea/stdlib/chunking.py, test/stdlib/test_chunking.py, mellea/stdlib/__init__.py (covered by #923), and the PartialValidationResult class in mellea/core/requirement.py (covered by #924). Those files appear in this diff because the branch is stacked, not because this PR changes them.

What to focus on:

• mellea/core/requirement.py — the stream_validate() method added to Requirement (one new method, ~10 lines)
• test/core/test_stream_validate.py — 7 new unit tests

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Summary

Adds async stream_validate(chunk, *, backend, ctx) -> PartialValidationResult to the base Requirement class as a per-chunk streaming validation hook. The default implementation returns PartialValidationResult("unknown") — subclasses override to inspect accumulated chunks and signal "pass" or "fail" early. The method is explicitly documented as non-mutating; state isolation is the orchestrator's responsibility.

Design decisions

Per the agreed Phase 1 spec:

• Method name: stream_validate (not avalidate)
• Return type: PartialValidationResult (tri-state: "pass", "fail", "unknown") — introduced in feat(core): add PartialValidationResult with tri-state semantics #898 / feat(core): add PartialValidationResult with tri-state semantics #924
• backend and ctx are keyword-only: prevents positional confusion and makes future parameter additions non-breaking
• "pass" is informational: does not short-circuit the final validate() call in Phase 1
• No reset() method: state management is delegated to the orchestrator via cloning

No LLM-as-a-Judge logic is added here. This is a pure hook for custom validation overrides.

Test plan

• test_default_returns_unknown — base class always returns "unknown"
• test_default_returns_partial_validation_result_instance — correct return type
• test_stream_validate_is_coroutine — method is async
• test_subclass_can_return_pass — subclass override returning "pass" works
• test_subclass_can_return_fail — subclass override returning "fail" with reason works
• test_does_not_mutate_requirement — calling the method leaves self unchanged
• test_stream_validate_idempotent — multiple calls leave self unchanged
• Full cumulative test suite (Wave 1 + Wave 2): 52 passed

Related

Closes #900. Part of streaming epic #891. Stacks on #898 / #924 and #899 / #923.