fix(spec): correct enum values in validation error example

[matanweks]

Summary

The validation error example in Section 6.5 has two inconsistencies:

1. Mismatched error reference: The request sends status=TASK_STATE_RUNNING but the error message references 'running' — updated to reference 'TASK_STATE_RUNNING' to match the actual input
2. Lowercase enum values in error message: The valid values were listed as pending, working, completed, failed, canceled but Section 5.5 mandates SCREAMING_SNAKE_CASE per the ProtoJSON specification — updated to list all valid TaskState enum values as defined in a2a.proto

Fixes #1642

Test plan

• Error message now references 'TASK_STATE_RUNNING', matching the request query parameter
• All 8 valid (non-UNSPECIFIED) TaskState enum values listed in SCREAMING_SNAKE_CASE
• Enum values verified against a2a.proto lines 187-208
• Consistent with Section 5.5 (JSON serialization rules) and other spec examples (e.g., line 1494: status=TASK_STATE_WORKING)

[gemini-code-assist]

Summary of Changes

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This pull request enhances the accuracy and consistency of a validation error example within the specification document. It addresses discrepancies in enum value referencing and formatting, ensuring the example adheres to the ProtoJSON specification and correctly reflects the defined TaskState enum values. This improves the clarity and correctness of the documentation for developers.

Highlights

• Corrected Mismatched Error Reference: Updated the validation error example to correctly reference 'TASK_STATE_RUNNING' in the error message, matching the actual input provided in the request.
• Standardized Enum Values: Adjusted the listed valid enum values in the error message to use SCREAMING_SNAKE_CASE, aligning with Section 5.5 of the ProtoJSON specification and including all 8 valid TaskState enum values.

Changelog

• docs/specification.md
  • Corrected validation error example for the 'status' field to use accurate enum values and SCREAMING_SNAKE_CASE formatting.

Activity

• Verified that the error message now correctly references 'TASK_STATE_RUNNING', matching the request query parameter.
• Confirmed that all 8 valid (non-UNSPECIFIED) TaskState enum values are listed in SCREAMING_SNAKE_CASE.
• Validated the enum values against the definitions in 'a2a.proto' lines 187-208.
• Ensured consistency with Section 5.5 (JSON serialization rules) and other specification examples.

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[gemini-code-assist]

Code Review

This pull request correctly fixes two inconsistencies in a validation error example within the specification, aligning the error message with the request and ensuring enum values adhere to the SCREAMING_SNAKE_CASE convention as mandated by the protocol. The change is accurate and improves the documentation's quality. I have added one comment highlighting a broader inconsistency in the specification regarding the format of HTTP error responses, which should be addressed for overall clarity.

[gemini-code-assist]

While this change correctly updates the enum values, the overall structure of this error example seems inconsistent with the error handling specification for the HTTP+JSON binding.

• Section 11.6 specifies that HTTP error responses should follow the AIP-193 format, which uses a top-level error object.
• This example (and the one in Section 6.4) appears to follow RFC 7807 Problem Details, as indicated by the Content-Type: application/problem+json header.

This creates a contradiction within the specification. To ensure clarity and consistency for implementers, it would be beneficial to align all HTTP error examples with the single format defined as normative in Section 11.6. This would likely require refactoring this entire error response example in a separate change.