fix(client): prevent duplicate Auth headers in SSE transport

[Jim1874]

Fixes SSE client spreading entire init object + explicit headers causing Headers merge issue with duplicate Auth.

Closes #1872

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🦋 Changeset detected

Latest commit: 0db63dc

The changes in this PR will be included in the next version bump.

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Name	Type
@modelcontextprotocol/client	Patch

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commit: 0db63dc