[test-improver] Improve tests for auth

[github-actions]

File: internal/auth/apikey_test.go

Changes

• Merged three overlapping format and length tests (IsValidHex, IsLowercaseHex, LengthConsistency) into one table-style TestGenerateRandomAgentID_Format with t.Run subtests.
• It uses assert.Regexp instead of regexp.MatchString plus assert.True.
• The uniqueness test now uses assert.NotContains on a map[string]struct{}.

Coverage: internal/auth was 96.8% before and is 96.8% after. The only uncovered line is the unreachable parse-error branch in ExtractAgentID, since ParseAuthHeader only errors on an empty header, which is handled earlier.

Verification: go vet, gofmt -l, and go test -count=3 -cover ./internal/auth/ all pass.

Generated by Test Improver · copilot · auto · 19.6 AIC · ⊞ 8.6K · ◷