Isolate skills client discovery in default-client test

[danbarr]

Summary

TestNewDefaultClient stubbed the TOOLHIVE_API_URL env reader but let the server-discovery step run against real local state. When a thv serve or Desktop server was running, discovery found it and returned its base URL (http://localhost for a Unix socket), so the "falls back to default URL when env is empty" and "applies options" subtests saw the discovered URL instead of http://127.0.0.1:8080 and failed. CI passed only because no server runs there, making this a confusing local-only failure.

newDefaultClientWithEnv already injects an env.Reader so the env-var step is testable; the discovery step just wasn't given the same treatment.

• Add a discoverFunc parameter to newDefaultClientWithEnv, mirroring the existing env.Reader injection. NewDefaultClient passes the real resolveViaDiscovery; no production behavior change.
• Stub discovery in the subtests so each resolution step (env var, discovery, default fallback) is tested in isolation, with no dependency on real local state.
• Add a subtest covering the discovered-server path, which was previously untested.

Type of change

• Bug fix

Test plan

• Unit tests (task test)
• Linting (task lint-fix)
• Manual testing (describe below)

Manual: ran go test -count=1 -run TestNewDefaultClient ./pkg/skills/client/; all four subtests pass. The discovery step is now fully stubbed, so the test is robust to a running local server by construction.

Does this introduce a user-facing change?

No. The change adds a test-injection seam; NewDefaultClient behaves identically.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 68.79%. Comparing base (5a8692d) to head (b2c456f).

Files with missing lines	Patch %	Lines
pkg/skills/client/client.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5417      +/-   ##
==========================================
+ Coverage   68.76%   68.79%   +0.02%     
==========================================
  Files         629      629              
  Lines       63922    63922              
==========================================
+ Hits        43958    43976      +18     
+ Misses      16707    16689      -18     
  Partials     3257     3257              

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