Fix | Render same-repo refs with remote Helm charts

[dag-andersen]

Summary

• Fix repo-server-api rendering for remote Helm charts whose $ref value files live in the same PR repository
• Pull the remote chart locally, stage same-repo refs under .refs, rewrite $ref value files to relative paths, and stream the temp tree to repo-server
• Keep external refs on the remote RPC path to preserve the existing issue Fix | Handle external ref sources when using repo-serve-api #428 behavior

Closes #441

Testing

• go build ./...
• go vet ./...
• go test ./pkg/reposerverextract/...
• make run-unit-tests

[jgarces-openspace]

FWIW tested this locally and it looks like it fixes the issue I ran into — ty!

Details (built from this branch with go build ./cmd, ran with --render-method repo-server-api against the private app-of-apps repo where I originally hit #441):

• On main (control): the affected app (remote chart from charts.jetstack.io + a $ref value file in the same private repo) still fails with repo server returned error for content source 1: … failed to get git client for repo https://github.com/<org>/<private-repo>.git and aborts the whole run.
• With this PR: the same run renders all apps (26 across both branches), exit 0.
• Ref values really come from the local checkout: a commit changing only the $ref'd value file (replicaCount 1→2 + a marker pod label) produces a diff showing exactly those changes on the remote-chart app — so the staged ref files are genuinely consumed, not silently skipped.
• Apps where a same-repo $ref feeds a local path: chart still render fine, so no regression on the Fix | Handle external ref sources when using repo-serve-api #428 behavior.