Isolate Studio sessions by identity

[yufeng201]

Description

Fix Studio session behavior across identity switches and legacy session scopes. This keeps session lists/messages cached per active identity, preserves readable legacy sessions, and prevents mutating operations from writing back into legacy scope.

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• Scope Studio session React Query keys by base URL, role, account, user, and key hash.
• Merge readable archived messages before live session messages and dedupe by message id.
• Preserve read-only fallback for legacy viking://session/{id} sessions while forcing writes to canonical user session scope.
• Generate stable Playground agent session IDs and make title generation language-neutral.
• Add backend and frontend tests for legacy fallback, session cache keys, archived messages, and Playground session IDs.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Checks run:

• git diff --check
• uv run pytest tests/service/test_session_service_metrics.py
• npm run test -- src/lib/sessions/api.test.ts src/lib/sessions/query-keys.test.ts src/routes/playground/-lib/utils.test.ts

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

N/A

Additional Notes

Split out from #2882 so reviewers can evaluate session/cache compatibility separately from console metrics/log access and performance chunk splitting.