[MOD-11888] Return descriptive errors from VecSimIndex_ResolveParams

[alonre24]

Summary

• Query-time parameter rejections (e.g. EPSILON, EF_RUNTIME, RERANK, hybrid policy) only ever surfaced a bare VecSimResolveCode, giving callers no way to explain why a specific parameter or value was rejected (e.g. the classic "Error parsing vector similarity parameters: Invalid option" with no further detail).
• VecSimIndex_ResolveParams gains an optional const char **err_msg out-param; each _ResolveParams_* helper now returns through a new SetResolveErr() helper that formats a specific message (e.g. "EPSILON is only valid for HNSW or SVS indexes") into a thread-local buffer instead of a bare error code.
• Adds a .claude/skills/vecsim-error-messages/SKILL.md documenting the pattern for future additions.

Index-creation-time error reporting (VecSimIndex_New returning NULL with no message) was considered under the same ticket but is descoped from this PR.

Test plan

• Existing unit tests (test_common, test_bruteforce, test_hnsw, test_svs) updated for the new VecSimIndex_ResolveParams signature and pass on CI
• check-format.sh / clang-format clean
• Follow-up (tracked separately under MOD-11888): wire RediSearch's VecSim_ResolveQueryParams to forward these messages into the client-facing error, and update RediSearch's tests accordingly

🤖 Generated with Claude Code

---

Note

Medium Risk
Breaking C API signature for VecSimIndex_ResolveParams requires all callers to pass the new argument; behavior on success is unchanged and err_msg is optional, but any missed call sites will fail to compile.

Overview
Query-time VecSim parameter resolution now exposes why a runtime param failed, not only a VecSimResolveCode.

VecSimIndex_ResolveParams takes an optional const char **err_msg. On failure, helpers route through new SetResolveErr() (printf into a thread-local 256-byte buffer) instead of returning bare codes. Messages name params via VecSimCommonStrings::*_STRING and describe constraints (wrong index type, query type, duplicate param, bad value, unknown param, hybrid policy conflicts). HYBRID_POLICY invalid values get a clearer message listing expected options.

A contributor skill doc (vecsim-error-messages) documents the pattern; RediSearch still uses canned errors and does not forward err_msg yet (follow-up under MOD-11888). Unit tests were updated for the new signature and assert representative error strings; nullptr for err_msg remains safe.

^{Reviewed by Cursor Bugbot for commit f7572c0. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.57%. Comparing base (ab6bb4f) to head (f7572c0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1051      +/-   ##
==========================================
+ Coverage   97.54%   97.57%   +0.03%     
==========================================
  Files         142      142              
  Lines        9405     9444      +39     
==========================================
+ Hits         9174     9215      +41     
+ Misses        231      229       -2     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.