Validate `operationId` uniqueness and warn on duplicates

[halotukozak]

Description

Per the OpenAPI 3.0 spec, operationId must be unique across all operations. If a spec contains duplicate operationId values, the generator may produce conflicting method names. While NameRegistry mitigates this by appending suffixes, users should be explicitly warned about the duplication so they can fix their spec.

Acceptance criteria

• SpecValidator detects duplicate operationId values across all operations
• A Warning is emitted for each duplicate, listing the conflicting path+method pairs
• Warnings are surfaced in ParseResult.warnings
• Code generation still succeeds (via NameRegistry deduplication) — this is a warning, not an error
• Tests cover specs with duplicate operationId values