feat(document): add GetOperationById to search operations across path…

[Mahdigln]

Description

Adds a GetOperationById(string operationId) method to OpenApiDocument that searches for an operation by its operationId across both Paths and Webhooks.

Closes #1654

Type of Change

• New feature (non-breaking change which adds functionality)

Related Issue(s)

Closes #1654

Changes Made

• Added GetOperationById(string operationId) method to OpenApiDocument
• Method searches both Paths and Webhooks to cover the full document
• Resolves operations through OpenApiPathItemReference (when the reference is resolved)
• Added GetOperationById entry to PublicAPI.Unshipped.txt

Testing

• Unit tests added/updated
• All existing tests pass

Tests cover:

• Returns matching operation from Paths
• Returns matching operation from Webhooks
• Returns null when not found
• Case-sensitive matching (getUser ≠ GetUser)
• Resolves operation through a $ref path item
• Returns first match when duplicate operationId exists (spec violation)
• Skips unresolved path item references gracefully
• Throws ArgumentNullException for null or empty operationId

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Versions applicability

• I have evaluated the applicability of my change against the other versions above.

Additional Notes

GetOperationByRef (resolving operationRef via JSON Pointer) is not included in this PR and can be addressed in a follow-up.