Feat/spend tracking

[gettingstuffdone]

Description:
This PR implements end-to-end query cost tracking and budget enforcement between the Go gateway and Python server:

• Gateway Cost Recording:
  • Updated the Proxy to capture response bodies using a bodyRecorder.
  • Supports recording spend from the X-Limnos-Cost-USD header (preferred) or by scraping est. $X.XX from the Markdown response
    (fallback).
  • Fixed a path routing bug where the /mcp prefix wasn't being correctly stripped before proxying to workers.
• Server Cost Reporting:
  • Added FastAPI middleware to automatically inject the X-Limnos-Cost-USD header into HTTP responses.
  • Implemented a ContextVar (current_query_cost) to safely communicate the estimated cost from the tool logic to the middleware
    across async boundaries.
• Testing:
  • Added server/tests/test_query_helpers.py to verify the cost ContextVar behavior.
  • Updated gateway/internal/mcp/proxy_test.go to support the new authenticated proxy signature.
• Dependency: This PR is based on feat/server-refactor.

Note: Since these branches are sequential, this PR will show the changes from the refactor as well unless feat/server-refactor is merged first.

[endemics]

Thanks!

[endemics]

There is a small linting issue with the golang code in the PR, would you be able to have a look and fix it? Thanks