feat: change default model from sonnet-4.5 to sonnet-4.6

[ambient-code]

Summary

• Update the default model for new sessions from claude-sonnet-4-5 to claude-sonnet-4-6 in the models manifest (models.json)
• Update the backend hardcoded fallback in sessions.go to use claude-sonnet-4-6
• Un-gate claude-sonnet-4-6 (featureGated: false) since it is now the platform default
• Update backend and frontend test fixtures to reflect the new default

Changes

File	Change
components/manifests/base/core/models.json	defaultModel and providerDefaults.anthropic → claude-sonnet-4-6; un-gate sonnet-4-6
components/backend/handlers/sessions.go	Hardcoded fallback model → claude-sonnet-4-6
components/backend/handlers/models_test.go	Test fixture and assertions updated for new default + model count
components/frontend/.../new-session-view.test.tsx	Mock data updated for new default
components/frontend/.../runner-model-selector.test.tsx	Mock data updated for new default

Test plan

• Frontend unit tests pass (631 passed)
• Backend unit tests pass (CI)
• Verify new sessions default to claude-sonnet-4-6 in the UI

Note: The public-api Docker build failure is a pre-existing issue on main (go mod download failure), unrelated to this PR.

Closes #1262

🤖 Generated with Claude Code

---

🤖 Ambient Session

[ambient-code]

This PR updates the default model for new sessions from claude-sonnet-4-5 to claude-sonnet-4-6 as requested in #1262.

Key changes:

• models.json: Updated defaultModel and providerDefaults.anthropic to claude-sonnet-4-6, and removed the feature gate on sonnet-4-6 so all users get it by default
• sessions.go: Updated the hardcoded fallback model used when no LLM settings are provided
• Tests: Updated all test fixtures and assertions across backend and frontend to reflect the new default

Frontend tests verified locally (631 passed). Backend/operator tests will be validated by CI.

🤖 Session

[Gkrumbach07]

@ambient-code fix unit tests

[jeremyeder]

Superseded by the fix/default-model-sonnet-4-6 branch.