[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 change for VecSimIndex_ResolveParams requires all callers to pass the new err_msg argument; query-param validation behavior is unchanged aside from richer optional messages.

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

VecSimIndex_ResolveParams gains an optional const char **err_msg out-parameter (documented in vec_sim.h). Rejection paths in vec_sim.cpp go through a new SetResolveErr() helper that formats into a thread-local 256-byte buffer; passing nullptr for err_msg remains safe. 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 now get a message listing expected batches / adhoc_bf instead of a bare code.

A .claude/skills/vecsim-error-messages/SKILL.md skill documents the pattern for future rejections. Unit tests (test_common, test_bruteforce, test_hnsw, test_svs) pass the new argument and assert specific err_msg strings where relevant.

Not in this PR: RediSearch still maps resolve codes to canned client errors and does not forward err_msg (noted as MOD-11888 follow-up).

^{Reviewed by Cursor Bugbot for commit d1c3eaa. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

❌ Patch coverage is 92.40506% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.52%. Comparing base (5d9c4fd) to head (d1c3eaa).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/VecSim/vec_sim.cpp	92.40%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1051      +/-   ##
==========================================
+ Coverage   97.40%   97.52%   +0.11%     
==========================================
  Files         141      141              
  Lines        8686     9050     +364     
==========================================
+ Hits         8461     8826     +365     
+ Misses        225      224       -1     

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