fix(workspaces): keep probe auth single-header

[Shrivats-Bogam]

What

The Workspace/credential "Test" button (probeAnthropic) could send two auth headers at once:

• Authorization: Bearer <key> and x-api-key: <ambient key> in bearer mode
• x-api-key: <key> and Authorization: Bearer <ambient token> in x-api-key mode

whenever ANTHROPIC_API_KEY / ANTHROPIC_AUTH_TOKEN exist in the environment. The @anthropic-ai/sdk falls back to those env vars when the matching constructor option is undefined (client.mjs:70-75), and the probe only set one option — the other came from the environment.

Dual auth is exactly what agent-probe.spec.ts pins against: gateways like MiniMax's international endpoint 401 ambiguous dual-header requests, and Anthropic's own API 401s OAuth-via-Bearer. The failure was reproducible on any machine with the env vars set (local repro confirmed both headers on the wire).

Fix

Pass the unused credential explicitly as null — the SDK's env fallback only applies when the option is undefined, so null keeps the wire carrying only the configured key:

• bearer mode: { authToken: key, apiKey: null }
• x-api-key mode: { apiKey: key, authToken: null }

Verification

• agent-probe.spec.ts: 6/6 pass locally (3/6 before the fix on a machine with ANTHROPIC_* env vars set)
• npx tsc --noEmit: clean
• Full pnpm test: 4014 passed / 2 failed / 39 skipped — both residual failures are environmental on Windows (symlink EPERM without Developer Mode; timing flake), unrelated to this change

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