feat(mcp): allow updating headers on MCPServerHTTP

[longcw]

Summary

• Expose MCPServerHTTP.headers as a property with a setter that propagates updates to the active httpx.AsyncClient.

Closes #5542

Usage

mcp_server = MCPServerHTTP(url=..., headers={"Authorization": f"Bearer {token}"})
await mcp_server.initialize()

# Later, when the token rotates:
mcp_server.headers = {"Authorization": f"Bearer {new_token}"}

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.

[theomonnom]

Lg! but let's deprecate MCPServer in favor of MCPToolset anyyway

[longcw]

let's deprecate MCPServer in favor of MCPToolset anyyway

I think we'll deprecate MCPServer for AgentSession but we still need to pass it to the MCPToolset right?

[toubatbrian]

This is an automated Claude Code Routine created by @toubatbrian. Right now it is in experimentation stage. The automation will start porting this PR into agents-js automatically.

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Triage result: skipping port

This PR adds a headers property setter on MCPServerHTTP that propagates updates to the live httpx.AsyncClient. The change is conceptually portable (an MCP runtime/plugin enhancement), but agents-js does not yet have an MCP server implementation — there is no equivalent MCPServerHTTP class, no agents/src/llm/mcp.ts, and no MCP-related exports in agents/src/llm/index.ts.

Porting this incremental enhancement in isolation isn't actionable without first porting the underlying MCP server infrastructure (transport selection between SSE / streamable HTTP, client lifecycle, tool listing/invocation, etc.), which is well beyond the scope of a single PR port.

No agents-js PR will be opened for this change. When MCP support lands in agents-js, the headers-setter behavior should be implemented at that time (the JS equivalent would expose headers as a getter/setter and update the underlying fetch/transport options on mutation).

cc @toubatbrian @livekit/agent-devs

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