fix(warnings): emit DUPLICATE_OPERATION as a structured warning

[yeongseon]

Summary

• WarningCode.DUPLICATE_OPERATION was dead code: defined but never emitted and untested. A METHOD path collision between two @openapi registrations silently dropped the earlier operation, appearing only as a logger.warning — so collect_spec_warnings(...) returned [] and --fail-on-warnings passed green while an endpoint vanished from the spec.
• Added a dedicated OpenAPIRegistry channel (add_duplicate_operation / duplicate_operations, cleared in clear()) that records each METHOD path collision during generation.
• _collect_duplicate_operation_warnings surfaces those records through collect_spec_warnings as a structured DUPLICATE_OPERATION SpecWarning, mirroring the discovery-skipped / binding-mismatch pattern.
• strict=True behaviour is unchanged (still raises OpenAPISpecConfigError).

Acceptance

• Structured DUPLICATE_OPERATION warning emitted on non-strict collision, in addition to the existing log.
• strict=True still raises.
• Test: duplicate registration yields exactly one DUPLICATE_OPERATION warning, last op wins, and --fail-on-warnings exits non-zero.
• make check-all green; coverage stays >= 95%.

Closes #386

[Copilot]

Pull request overview

Adds structured emission of WarningCode.DUPLICATE_OPERATION so METHOD/path collisions during spec generation are CI-observable via collect_spec_warnings(...) / --fail-on-warnings, rather than being log-only.

Changes:

• Record merge-time METHOD/path collisions in OpenAPIRegistry via add_duplicate_operation(...) and clear them in OpenAPIRegistry.clear().
• Surface recorded collisions as structured SpecWarning(code=DUPLICATE_OPERATION, ...) via _collect_duplicate_operation_warnings(...) and include them in collect_spec_warnings(...).
• Add tests covering structured duplicate-operation warnings, strict-mode behavior, and CLI --fail-on-warnings exit code behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/azure_functions_openapi/spec.py	Records duplicate METHOD/path collisions during generation and emits them as structured warnings via collect_spec_warnings.
src/azure_functions_openapi/registry.py	Adds a dedicated registry channel for duplicate operations, with deduping + sorting and clear() support.
tests/test_spec_warnings.py	Adds regression tests for structured duplicate-operation warnings, strict behavior, and --fail-on-warnings gating.

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[yeongseon]

Resolved rebase conflicts against main (empty-discovery + duplicate-operation warning channels now coexist in spec.py/registry.py). Addressed Copilot review: test_last_operation_wins_in_spec now asserts the surviving operation is the second registration (operationId == "post_second"), verifying "last wins" instead of merely "one survives". make check-all green.