Bump toolhive-core to v0.0.21 and use shared redis client

[reyortiz3]

Summary

toolhive-core v0.0.21 ships a shared redis/ package (introduced in toolhive-core#108) that consolidates the Redis connection layer — config types, TLS dialer, per-mode client construction, topology validation, and timeout defaults — that the auth-server's storage package and pkg/transport/session had each been carrying as a near-duplicate copy. Without this migration the next consumer (stacklok-llm-gateway) would have to land another copy and we'd keep paying the drift cost.

This PR migrates the auth-server half of that consolidation. pkg/transport/session is intentionally out of scope and will land in a follow-up.

What changed

• Bumped github.com/stacklok/toolhive-core to v0.0.21 (transitive bumps: go-redis/v9 → v9.19.0, miniredis/v2 → v2.38.0).
• pkg/authserver/storage/redis.go: removed local RedisConfig, SentinelConfig, RedisTLSConfig, ACLUserConfig, the Default*Timeout constants, and the buildTLSConfig / newTLSDialer / configureTLSDialer / validateConfig helpers. NewRedisStorage now takes tcredis.Config plus keyPrefix and delegates topology validation, defaults, TLS plumbing, and Ping verification to the shared package. Auth-server-specific invariants (ACL password required, key prefix required) stay in the constructor.
• pkg/authserver/runner/embeddedauthserver.go: convertRedisRunConfig produces a tcredis.Config; topology checks that duplicated tcredis's Validate were dropped — those errors now surface from the shared package.
• Tests: deleted pkg/authserver/storage/redis_tls_test.go (now exercising dependency code per the testing rule), trimmed the validation table down to the two auth-server-specific cases, table-drove the three connection-failure tests, removed runner-side topology validation subtests, and updated env-var subtests to the new field layout (cfg.Username / cfg.Password / cfg.DB).

Net: +146 / -805 lines.

Type of change

• Refactor

Test plan

• `go build ./...` — clean
• `go vet ./...` — clean
• `task lint-fix` — 0 issues
• `go test ./pkg/authserver/... -race -count=1` — all packages pass

Does this introduce a user-facing change?

No. Connection-mode behavior, TLS behavior, and configuration schema (`RedisRunConfig` JSON/YAML fields) are unchanged. Error messages for topology misconfiguration now come from the shared package (`redis: invalid configuration: ...`) instead of the auth-server's local validator.

Special notes for reviewers

• The shared package validates topology, applies timeout defaults, and Pings before returning. NewRedisStorage no longer wraps the tcredis error — the `redis: ...` prefix already makes its origin clear.
• Two test cases were removed because they tested topology validation that's now owned by toolhive-core (and exercised in its own tests). One createStorage subtest that asserted on the same topology error path was also removed — re-asserting it here would have required either t.Setenv inside a t.Parallel() parent or duplicating tcredis's test surface.
• Pre-existing unused write and `rangeint` diagnostics in the touched test files are unrelated to this refactor.

Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 87.09677% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.40%. Comparing base (4bc4624) to head (b9c3d1b).

Files with missing lines	Patch %	Lines
pkg/authserver/runner/embeddedauthserver.go	81.81%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5318      +/-   ##
==========================================
+ Coverage   68.38%   68.40%   +0.01%     
==========================================
  Files         620      620              
  Lines       63432    63316     -116     
==========================================
- Hits        43378    43309      -69     
+ Misses      16819    16774      -45     
+ Partials     3235     3233       -2     

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