fix(client): preserve custom Accept headers in StreamableHTTPClientTransport

[nielskaspers]

Summary

StreamableHTTPClientTransport unconditionally overwrites custom Accept headers provided via requestInit.headers with SDK defaults. This change only sets the default Accept header when the user has not already provided one.

Issue

Fixes #1646

Changes

• In send() (POST requests): only set accept: application/json, text/event-stream if no Accept header exists
• In _startOrAuthSse() (GET SSE requests): only set accept: text/event-stream if no Accept header exists

Testing

• Added test: custom Accept header is preserved on POST requests
• Added test: custom Accept header is preserved on GET SSE requests
• Added test: default Accept header is set when none provided
• All 260 existing tests continue to pass
• Lint passes

[changeset-bot]

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Latest commit: 21b050f

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