Add Windows Claude Desktop credential helper

[stantheman0128]

Summary

thv llm setup --client claude-desktop detected Claude Desktop on Windows and then failed with claude-desktop LLM gateway setup is not supported on Windows yet. The helper it would have written is a POSIX /bin/sh script (claude-desktop-helper.sh).

The Windows early return is gone. Setup now writes claude-desktop-helper.cmd. The shim still execs the absolute thv path from os.Executable() (GUI-launched Claude Desktop has no useful PATH). It uses goto instead of a parenthesized if so a ) in the path cannot break the block, and call so a .cmd token helper returns. % and " in the path are escaped for cmd.exe.

Fixes #6507

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation
• Other (describe):

Test plan

• Unit tests (go test ./pkg/client/ -count=1 -timeout 180s -run "CredentialHelper|QuoteFor|WriteCredential")
• E2E tests (task test-e2e) (suite skips on Windows; POSIX fake-browser)
• Linting (task lint-fix)
• Manual testing (describe below)

Isolated %USERPROFILE% on this Win11 box (real Claude store untouched):

thv llm setup --client claude-desktop --lazy --gateway-url https://127.0.0.1:9 --issuer https://127.0.0.1:9 --client-id dummy-self-found --tls-skip-verify

Setup exit 0. Wrote .toolhive\llm\claude-desktop-helper.cmd with call "<abs thv.exe>" llm token. Config document inferenceCredentialHelper pointed at that .cmd. thv llm teardown claude-desktop removed the shim and the config document.

Did not launch Claude Desktop itself, so the GUI helper invoke is untested here.

API Compatibility

• This PR does not break the v1beta1 API, OR the api-break-allowed label is applied and the migration guidance is described above.

Does this introduce a user-facing change?

Yes. thv llm setup --client claude-desktop now configures Claude Desktop on Windows.

Special notes for reviewers

Assigned on #6507. Direct-mode clients still use the PATH-resolved thv llm token shell string; only Claude Desktop needs the absolute-path shim.

Assisted by Cursor.

Evidence

go test ./pkg/client/ -count=1 -timeout 180s -run "CredentialHelper|QuoteFor|WriteCredential"
ok  	github.com/stacklok/toolhive/pkg/client	1.560s

Live setup/teardown under a temp USERPROFILE: setup exit 0, .cmd present, teardown removed the shim. Real %LOCALAPPDATA%\Claude-3p file count stayed 0.

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.24%. Comparing base (e30f1bf) to head (e349867).

Files with missing lines	Patch %	Lines
pkg/client/llm_gateway_credential_helper.go	25.00%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6516      +/-   ##
==========================================
- Coverage   78.25%   78.24%   -0.01%     
==========================================
  Files         770      770              
  Lines       75521    75542      +21     
==========================================
+ Hits        59100    59111      +11     
- Misses      16416    16426      +10     
  Partials        5        5              

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