fix(types): reject multiple content oneof arms in Part.fromJSON

[Varun-S10]

Description

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Fixes #643 🦕.

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What was the bug?

In the A2A protobuf specification, a Part message has a content field defined as a oneof (text, raw, url, data), meaning only one of these fields can be present at a time.

Previously, if incoming JSON contained multiple content fields (for example { text: "hello", url: "https://example.com/x" }):

• Part.fromJSON did not throw an error.
• It selected "text" and silently dropped "url" with zero warning or notice.

In contrast, a2a-python and the proto3 JSON specification treat multiple oneof fields as malformed input and reject them with a parse error.

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How is it fixed?

1. src/types/pb/a2a.ts:
   • Added validation inside Part.fromJSON to verify that at most one content oneof field is present.
   • Throws an error (Message type "lf.a2a.v1.Part" should not have multiple "content" oneof fields: ...) when multiple arms are supplied.
2. test/types/part_oneof.spec.ts:
   • Added unit tests covering all single valid arms (text, raw, url, data), empty content, and multiple arm combinations.

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Verification

• Part.fromJSON throws on multiple oneof fields.
• Unit tests in test/types/part_oneof.spec.ts passed (8/8).
• Full test suite (npm test) passed (69 test files, 1,495 tests).
• ESLint and TypeScript checks (npm run lint) passed with 0 errors.
• Build checks (npm run build and npm run test-build) passed.

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[bartek-gralewicz]

Any changes to the src/types/a2a.ts are not permitted. This file is autogenerated based on the proto recipe. Please see the first line of the file: src/types/a2a.ts:1