fix(schema): honour additionalProperties: true at schema compilation

[Muizzkolapo]

Summary

compile_unified_schema (agent_actions/output/response/vendor_compilation.py) hardcoded "additionalProperties": False for the OpenAI-compatible, Anthropic, and Ollama targets, ignoring a schema file's declared additionalProperties: true. A UDF (kind: tool) or LLM output carrying an extra key then passed preflight but crashed mid-run with Additional properties are not allowed — after upstream LLM tokens were already spent, with downstream cascade-skips.

The fix reads the declared value at each of the three compile sites:

"additionalProperties": unified.get("additionalProperties", False),

• One fix, both validators. The compiled schema feeds both UDF-output validation (_validate_against_schema) and LLM-output validation, so fixing the compiler fixes both paths.
• Default stays strict. When the schema file omits the key, the compiled schema still emits False — no behaviour change for existing schemas (7897 existing tests unchanged).
• Nested-items sibling verified, not edited. expander_schema.py:83 uses setdefault("additionalProperties", False) on the source item dict, which already preserves a declared true. Left unchanged; pinned with a regression test so a future refactor can't silently regress it.
• Provider caveat (documented, not blocked): OpenAI structured-outputs strict mode requires additionalProperties: false. A user who declares true and targets strict OpenAI opts into the provider's behaviour — that is the author's explicit choice; the framework honours the file.

Scope: only the three hardcoded False sites changed. The gemini/cohere branches (already omit the key) and the jsonschema validator (correctly enforces whatever it's handed) are untouched.

Verification

• RED→GREEN: tests/unit/output/response/test_additional_properties_honoured.py (new). The five vendor honour-true cases + extra-key-validates were RED against the hardcode; GREEN after the fix. Default-strict guards and the nested-items pinning guards are green throughout.
• End-to-end: test_udf_output_extra_key_accepted_when_true compiles a additionalProperties: true schema and runs an extra-key payload through the real _validate_against_schema UDF path — no crash; the default schema still raises SchemaValidationError.
• pytest tests/unit/output tests/validation tests/core/test_udf_schema_validation.py -q → 542 passed.
• Full suite: pytest -q → 7897 passed.
• ruff check agent_actions tests and ruff format --check agent_actions tests → clean.