fix(types): validate RFC 3339 timestamp and normalize to UTC in codecs

[Varun-S10]

Description

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Fixes #641 🦕

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What was the bug?

In the A2A protobuf specification, TaskStatus.timestamp and ListTasksRequest.status_timestamp_after are typed as google.protobuf.Timestamp. According to the proto3 JSON specification:

1. Timestamps must be valid RFC 3339 formatted strings.
2. Timezone offsets (such as +05:30 or -05:00) must be normalized to UTC (Z).
3. Malformed non-timestamp inputs must be rejected with an error.

Previously, TaskStatus.fromJSON and ListTasksRequest.fromJSON passed timestamp inputs straight through as raw strings using globalThis.String(object.timestamp) without validation or normalization:

• Malformed inputs like "not-a-timestamp", "", or 12345 were accepted without error and forwarded downstream, causing a2a-python peers to crash when receiving them.
• Timezone offsets (e.g., "2026-01-01T05:30:00+05:30") were never converted to UTC ("2026-01-01T00:00:00Z"), leading to byte-level mismatches during card signing and canonicalization.

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How is it fixed?

1. src/types/pb/a2a.ts:
   • Added fromJsonTimestamp helper that validates RFC 3339 date strings, throws a descriptive error on invalid inputs ("not-a-timestamp", "", 12345, boolean, object), and normalizes non-UTC timezone offsets to UTC (Z).
   • Updated TaskStatus.fromJSON and ListTasksRequest.fromJSON to use fromJsonTimestamp.
2. test/types/timestamp.spec.ts:
   • Added 14 unit test cases covering already-normalized UTC strings, positive/negative offset normalization, subsecond preservation, Date instances, and strict rejection of invalid values.

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Verification

• TaskStatus.fromJSON and ListTasksRequest.fromJSON throw on malformed/empty/numeric timestamps.
• Non-UTC timezone offsets normalize to UTC "2026-01-01T00:00:00Z".
• Unit tests in test/types/timestamp.spec.ts passed (14/14).
• Full test suite (npm test) passed (69/69 test files, 1,501 tests).
• ESLint and TypeScript checks (npm run lint) passed with 0 errors.
• Build and bundle checks (npm run build && npm run test-build) passed cleanly.

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