Fix v2 API 404 when a /config request is proxied to another node

[epugh]

Summary

• Solr forwards a v2 request to the node that actually hosts the target collection using the short /c/{collection}/... path alias (see HttpSolrCall#sendRemoteProxy), but Constants.INDEX_PATH_PREFIX's JAX-RS @Path regex only recognized cores or collections, not c. Any JAX-RS resource built on this prefix therefore 404'd with Cannot find API for the path whenever a request had to be proxied cross-node.
• Widened the regex to accept c as an alias for collections.
• This was discovered while migrating GET /config/requestHandler to JAX-RS (SOLR-16396): PackageToolTest uses a 2-node cluster and intermittently hit this exact 404 whenever its HTTP client landed on the node that didn't host the test collection's replica. Confirmed the bug is general (not specific to that migration) by reproducing it against an already-merged, unrelated endpoint (GET /config/overlay).

Test plan

• Added GetConfigSolrCloudTest.testGetOverlayFromCollectionViaNonHostingNode, which bumps the test cluster to 2 nodes and deliberately sends a request to the node that does not host the collection's replica, forcing an internal proxy. Verified it fails with Cannot find API for the path: /config/overlay before the fix, and passes after.
• Full GetConfigSolrCloudTest suite passes.
• org.apache.solr.handler.admin.api.*, org.apache.solr.handler.admin.*, org.apache.solr.api.*, org.apache.solr.jersey.* (436 tests) pass.
• org.apache.solr.cloud.api.collections.*, org.apache.solr.handler.*V2*, org.apache.solr.servlet.* pass.
• spotlessApply — no formatting changes needed.

🤖 Generated with Claude Code

https://claude.ai/code/session_01RzzUgiebQ8ZhhgE2E7ZZU5

[epugh]

@jaykay12 this is the actual issue you were bumping into with package tool test!!!

[epugh]

@gerlowskija would love your review. Also should we just drop the /c/ thing???

[epugh]

This was discovered while reviewing/debugging #5168 (SOLR-16396: migrate `GET /config/requestHandler` to JAX-RS) — its `PackageToolTest` was intermittently failing with `Cannot find API for the path: /config/requestHandlers` whenever the test's 2-node cluster routed the HTTP request to the node that didn't host the test collection's replica. Traced it down to this proxy-routing gap, confirmed it's general (not specific to that migration) by reproducing it against the already-merged `GET /config/overlay` endpoint here, and split the fix out into its own PR since it's independent of that migration.

[epugh]

/no-changelog

[dsmiley]

I wonder do we really need this justification. It doesn't look strange for 'c' to be here (to me); I'm not going to wonder what would break if one of these items was missing.

[sigram]

Looks good to me. Claude Opus 5.5 has this to say:

Real but minor effects:

1. The alias covers every resource using the shared prefix, not just /config. That is Select, GetSchema, UpdateSchema, Config and Tasks, and each becomes reachable directly at /api/c/{coll}/.... I think that's the intended behaviour, but the PR title only mentions /config, so it's worth saying in the PR description and changelog.
2. The test class now starts a 2-node cluster for every test, which is slightly slower. That's acceptable.

Nits:

• The comment at solr/webapp/web/js/angular/services.js:266 still says (cores|collections).

Result: authorization, proxy dispatch and generated clients are unaffected, and the c alias now applies to all five resources using the shared prefix. Remaining items are minor: an incomplete comment, and the PR title understating the scope.