Fix subscription sessions showing API cost estimates

[alejandro-ao]

Summary

• Mark assistant usage produced through subscription-backed OAuth credentials.
• Exclude API catalog rates from TUI, RPC model metadata, and HTML usage estimates for subscription requests.
• Preserve catalog estimates for direct API-key requests, including Anthropic API-key usage.
• Add regression tests and document the behavior.

User experience

Subscription sessions no longer display misleading API-equivalent dollar costs. Token, cache, timing, and tool usage remain available; API-key sessions retain their estimated costs.

Issue

Addresses the reported issue where subscription-backed sessions showed direct API pricing. No tracked issue number was provided.

Manual validation

1. Log in to an OAuth/subscription provider such as OpenAI Codex or Anthropic.
2. Run a prompt with enough usage to populate the session sidebar.
3. Confirm the session does not show an API-equivalent dollar estimate ($N/A is expected when no provider-reported charge exists).
4. Repeat with a direct API key and confirm catalog-based estimates still appear.
5. Optionally export both sessions and verify the Usage tab follows the same behavior.

Verification

• uv run pytest — 1925 passed, 3 skipped
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy
• uv build

[alejandro-ao]

Implemented the review fix in eb5a75f.

• Persist both api and subscription pricing modes on new assistant responses.
• Keep historical API pricing stable after switching the active credential to OAuth.
• Added regression coverage and updated the development note.

Validation:

• uv run pytest — 1925 passed, 3 skipped
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy
• uv build