Cache plan preview repository routing

[rootp1]

Summary

• cache plan preview repository-to-piped matches by project, repo remote, and base branch
• avoid repeated full active-piped scans for repeated requests on the same route
• add focused tests and a benchmark covering 1, 100, and 10000 pipeds

Linked Issue

Fixes #7087

What Changed

• added a bounded in-memory TTL cache for RequestPlanPreview routing matches
• reused cached repository matches before falling back to pipedStore.List
• added regression tests for cached matches and cached empty results
• added a benchmark verifying repeated requests stay off the datastore scan path

Verification

• go test ./pkg/app/server/grpcapi -run 'TestRequestPlanPreview|TestRequireAPIKey' — passed
• go test -bench BenchmarkRequestPlanPreviewRepositoryRouting -benchmem ./pkg/app/server/grpcapi — passed
• make build/go — passed
• make check/gen/code — passed
• make check — not passed: lint/go uses a pinned golangci-lint image built with Go 1.25, but this repository targets Go 1.26.2, so lint fails before the rest of make check can complete
• make check/dco — not passed: fails on upstream commit 25ae69937e55e8330bff96c34346718f3e5e7148, which is already contained in upstream/master

Scope

• only the plan preview routing path and its focused tests were changed; unrelated files were not modified

What this PR does:

Caches repository routing for plan preview requests so repeated requests do not rescan every active piped in the project.

Why we need it:

Plan preview is an interactive path, and repeated requests for the same repository and branch should not require a full active-piped scan each time.

Which issue(s) this PR fixes:

Fixes #7087

Does this PR introduce a user-facing change?:

• How are users affected by this change: Plan preview routing avoids repeated full scans for the same repository and base branch, reducing control-plane work on repeated requests.
• Is this breaking change: No
• How to migrate (if breaking change): Not applicable