fix: handle invalid selector errors

[alxckn]

This PR

Invalid selector expressions in the flagd-selector header were causing unhandled errors that surfaced as 500 responses. This PR validates the selector at parse time and maps invalid input to the appropriate error code
(InvalidArgument / HTTP 400) across all service layers (flag evaluation, flag sync, OFRep).

As part of this fix, the Selector construction API has been redesigned to eliminate unchecked string-based construction entirely:

• NewSelector(expr string) now returns an error for unknown keys
• WithIndex is removed (breaking change) — replaced by typed methods WithSource and WithFlagSetId which cannot produce an invalid selector

Related Issues

Notes

Breaking change: (Selector).WithIndex(key, value string) has been removed from the public API. Callers should migrate to Selector{}.WithSource(source) or Selector{}.WithFlagSetId(id).

(with AI help)

[netlify]

✅ Deploy Preview for polite-licorice-3db33c canceled.

Name	Link
🔨 Latest commit	9dac26e
🔍 Latest deploy log	https://app.netlify.com/projects/polite-licorice-3db33c/deploys/6a1dfe659edc500008b2746b

[gemini-code-assist]

Code Review

This pull request refactors the Selector creation and manipulation. NewSelector now validates selector keys and returns an error for invalid keys, with corresponding error handling added across evaluation and sync services. Additionally, generic WithIndex calls are replaced with type-safe methods like WithSource and WithFlagSetId. The review feedback recommends two improvements: recording the error on the tracing span when startResolveV2 fails in flag_evaluator_v2.go, and setting exitReason = "error" when returning an invalid selector error in flag-sync/handler.go to ensure accurate metrics.

[toddbaert]

@alxckn unit test for NewSelector is good, but I'd like service-level coverage too; the whole point is "no more 500s on bad selectors". One test per surface asserting:

• connect.CodeInvalidArgument on v0/v1/v2 eval + sync
• HTTP 400 on OFREP (single + bulk)

Otherwise this can regress silently. WDYT?

Also FYI, some of the sonar issues should be fixed on main now; might be worth a rebase/merge before the next push.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
1.9% Duplication on New Code

See analysis details on SonarQube Cloud

[alxckn]

@toddbaert thanks for the review. I took care of comments, added integration tests to prevent regressions, and the sonar warnings seem resolved 👍