[FEATURE]: add dynamic MCP headersHelper support for remote authentication #27161

Summary

Add native support for dynamic authentication headers for remote MCP servers.

The main use case is short-lived bearer tokens or other custom auth flows that cannot be represented cleanly with static headers and do not fit the existing OAuth path.

Proposed shape

Add a structured headersHelper object on remote MCP servers, for example:

{
  "headersHelper": {
    "command": ["node", "./scripts/mcp-headers.js"],
    "environment": {
      "PROFILE": "prod"
    },
    "timeout": 10000
  }
}

Proposed behavior

• run the helper on connect and reconnect
• no caching in v1
• helper stdout must be a JSON object of string headers
• helper-produced headers override static headers
• require trust approval before executing project-local or remote-config helpers

Why this would help

Today OpenCode supports static headers and OAuth, but some MCP servers need custom auth headers generated at runtime.

Notes

I have a local implementation draft for this, but I do not want to open a PR until the core team confirms the design is acceptable.

If this direction looks good, I can open a focused PR with the code, tests, docs, and generated SDK/OpenAPI updates.