fix: add twirp catch-all with proxy support for unmatched paths (#237)

[yinheli]

GitHub Actions Cache Server only has Twirp routes for CacheService. Unmatched paths (e.g. ArtifactService) under /twirp/ return 404 because Nitro marks the segment as handled but no catch-all exists inside it.

Changes:

• Add routes/twirp/[...path].ts — proxies unmatched Twirp paths to DEFAULT_ACTIONS_RESULTS_URL, falls back to stub 200 on failure
• Add lib/proxy-agent.ts — reads HTTP_PROXY/HTTPS_PROXY/NO_PROXY env vars and creates an undici ProxyAgent; inject via fetchOptions.dispatcher
• Add undici as a direct dependency (global fetch uses Node.js built-in undici which is incompatible with the locally-installed ProxyAgent)
• Strip content-length/host/connection from forwarded headers to avoid undici header validation errors during CONNECT tunneling

[DrJume]

Thank you for your contribution.
Please submit a failing test, which this PR is aiming to resolve, so we can verify the issue and prevent future regressions.

[yinheli]

@DrJume Thanks for the review!

Here's the failure context and logs that motivate this PR.

Our CI failure (artifact upload step):

Attempt 1 of 5 failed with error: Failed request: (500) Server Error. Retrying request in 3000 ms...
Attempt 2 of 5 failed with error: Failed request: (500) Server Error. Retrying request in 5246 ms...
Attempt 3 of 5 failed with error: Failed request: (500) Server Error. Retrying request in 7013 ms...
Attempt 4 of 5 failed with error: Failed request: (500) Server Error. Retrying request in 10395 ms...

Warning: Failed to CreateArtifact: Failed to make request after 5 attempts: Failed request: (500) Server Error

This happens during the Post Build and push image step when the runner tries to upload the docker build record as an artifact.

Reproduction from #237:

$ curl -i -X POST -H 'Content-Type: application/json' --data '{}' \
    http://cache-server:3000/twirp/github.actions.results.api.v1.ArtifactService/CreateArtifact

HTTP/1.1 404 Not Found
{"statusCode":404,"statusMessage":"Cannot find any route matching /twirp/github.actions.results.api.v1.ArtifactService/CreateArtifact."}

Our environment shows 500 instead of 404 (likely because the @actions/artifact client retries and the aggregated failure appears as 500), but the root cause is the same: unmatched /twirp/* paths are not proxied to DEFAULT_ACTIONS_RESULTS_URL.