Reject percent-encoded parent traversal in credential broker paths

[yassine-ceo]

Closes the Git Broker finding from #125.

Problem

The credential broker and the git HTTP broker gate upstream paths through brokerPathAllowed, which matches the raw pathname against allowedPathPrefixes. Percent-encoded parent segments (..%2f, %2e%2e%2f) survive URL parsing with the encoded bytes intact, so a path like /acme/repo.git/..%2fadmin still starts with the allowed prefix while upstream servers decode it to /admin — a path traversal that bypasses the allowlist.

Fix

brokerPathAllowed now rejects any pathname containing a .. segment once percent-decoded (hasParentSegment), fail-closed on malformed escapes. This sits in the shared gate both brokerCredentialCall and brokerGitHttp flow through, so both entry points are covered by one check.

Tests

• test/credential-broker.test.ts: encoded traversals (..%2f, %2e%2e%2f, %2e%2e%2f variants) refused with path_not_allowed, zero upstream calls.
• test/git-http-broker.test.ts: encoded traversals in the git broker path refused with path_not_allowed, no upstream git fetch.

Verified: npm run typecheck, npm run lint (oxlint), and both affected test suites pass.

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[rajpratham1]

The implementation addresses a legitimate path traversal concern, applies the validation consistently, and includes targeted regression tests that verify both rejection behavior and the absence of unintended upstream requests. I don't see any blocking issues in the proposed changes.