fix(client): add Accept header in SSEClientTransport.send()

[ss19901101]

Problem

SSEClientTransport's send() method was missing the Accept header when making POST requests to MCP servers. This caused compatibility issues with MCP servers that strictly require the Accept: application/json, text/event-stream header for Streamable HTTP transport.

Solution

Added the Accept header in SSEClientTransport.send() method to match the behavior of StreamableHTTPClientTransport:

headers.set('accept', 'application/json, text/event-stream');

Testing

• Verified the fix resolves connectivity issues with Zhipu's MCP server (https://open.bigmodel.cn)
• Before: Server returned error "Accept header must include both application/json and text/event-stream"
• After: Server accepts the request and returns proper SSE response

Comparison

Transport	Before	After
StreamableHTTPClientTransport	✅ Accept header present	✅ No change
SSEClientTransport	❌ Missing Accept header	✅ Accept header added

This aligns the two transports and improves MCP server compatibility.

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