fix(client): send MCP standard POST headers

[he-yufeng]

Fixes #2176.

This makes the streamable HTTP client include the SEP-2243 standard POST headers for non-batch JSON-RPC requests:

• Mcp-Method is set from the JSON-RPC method name.
• Mcp-Name is set from params.name or params.uri when present.
• Batch requests keep the existing behavior.

Validation:

• pnpm --filter @modelcontextprotocol/client test -- streamableHttp.test.ts
• pnpm --filter @modelcontextprotocol/client typecheck
• pnpm --filter @modelcontextprotocol/client lint
• pnpm --filter @modelcontextprotocol/client build
• git diff --check
• pre-push hook: full repo lint, build, and typecheck

[changeset-bot]

🦋 Changeset detected

Latest commit: 487fd15

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

This PR includes changesets to release 1 package

Name	Type
@modelcontextprotocol/client	Patch

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commit: 487fd15

[caomengxuan666]

Code looks correct for the single-request happy path. A few test coverage gaps I'd like to see addressed before merge:

1. Notification test: Send a notification like {jsonrpc: "2.0", method: "notifications/initialized"} and verify Mcp-Method is set to notifications/initialized and Mcp-Name is absent. Per SEP-2243, notifications also require Mcp-Method.

2. Response test: Send a JSON-RPC response {jsonrpc: "2.0", id: "1", result: {}} and verify neither Mcp-Method nor Mcp-Name is set — the guard on 'method' in message should skip it.

3. Batch test: The existing 'should send batch messages' test doesn't assert header behavior. Send a JSONRPCMessage[] and verify mcp-method is not set.

4. No-params test: Send {jsonrpc: "2.0", method: "initialize", id: "1"} (no params) and verify Mcp-Method is set but Mcp-Name is absent.

The implementation itself is solid — the ternary with typeof guard is the right call over ?? for the mcpName assignment. Just needs the tests to document the edge cases.

[he-yufeng]

Rebased this onto current main and pushed the refreshed branch.

I also checked the current head against the requested coverage points. The PR now has focused Streamable HTTP client tests for:

• JSON-RPC notifications including the MCP method header
• JSON-RPC responses not including MCP method/name headers
• JSON-RPC batch requests not including MCP method/name headers
• requests without params omitting the MCP name header

Local validation:

pnpm --filter @modelcontextprotocol/client test -- streamableHttp.test.ts
# 58 passed

pnpm --filter @modelcontextprotocol/client run typecheck
pnpm --filter @modelcontextprotocol/client run lint
git diff --check

The pre-push hook also completed after reinstalling the workspace links:

pnpm -r build
pnpm -r typecheck
pnpm -r lint