fix: add Transfer-Encoding header to SSE responses for HTTP/2 compati…

[tinynakji]

…bility

Adds Transfer-Encoding: chunked header to all SSE response headers in WebStandardStreamableHTTPServerTransport to prevent HTTP/2 protocol errors.

Motivation and Context

Some HTTP adapters (e.g., @hono/node-server) buffer small SSE responses and automatically add a Content-Length header. When the SSE stream closes, this causes an HTTP/2 PROTOCOL_ERROR because HTTP/2 doesn't allow Content-Length with streaming responses.

Adding Transfer-Encoding: chunked prevents this buffering behavior and ensures SSE streams work correctly over HTTP/2.

Fixes #1619

How Has This Been Tested?

• Ran all existing streamableHttp tests across packages:
  • @modelcontextprotocol/server: 28 tests passed
  • @modelcontextprotocol/client: 38 tests passed
  • @modelcontextprotocol/node: 73 tests passed

Breaking Changes

None. This is an additive change to response headers.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Affected locations in packages/server/src/server/streamableHttp.ts:

• handleGetRequest method (~line 450)
• replayEvents method (~line 507)
• handlePostRequest method (~line 757)

Reference: https://github.com/honojs/node-server/blob/main/src/listener.ts#L463-L491

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commit: 07d76e4