fix(coding-agent): isolate invalid provider registrations

[mrrobotke]

Summary

• treat failed extension provider registrations as non-fatal warnings
• keep valid providers available when another extension has no resolvable credential
• add focused regression coverage for mixed unavailable and valid providers

Behavior

An invalid provider is skipped with a diagnostic that identifies its extension and validation error. Other provider registrations continue loading, while selecting the skipped provider still follows the existing unavailable-provider path.

Validation

• npx tsx ../../node_modules/vitest/dist/cli.js --run test/agent-session-services.test.ts test/suite/regressions/1230-provider-registration-isolation.test.ts
• npm run check

Fixes #1230

Note

Isolate failed provider registrations in coding agent so valid providers still load

Previously, a single extension with an unavailable provider credential could block other providers from loading. Now, failed provider registrations in createAgentSessionServices are caught individually and reported as warnings ("Skipped provider from extension...") instead of errors, leaving valid providers unaffected. A regression test in 1230-provider-registration-isolation.test.ts verifies that a missing-credential provider is skipped while a valid provider registers successfully.

^{Macroscope summarized 52add7f.}

[jonaowen]

The registration loop already isolates each extension provider structurally; changing a rejected provider registration from a process-fatal runtime diagnostic to a warning completes that isolation. The regression proves the invalid model is absent, a later valid provider remains registered, and no fatal diagnostic survives. Other extension load errors remain unchanged.