Update AI docs for retired hosted models (#49486)

## Summary
- Update hosted model and context-window tables in docs/src/ai/models.md
to remove retired models and list current replacements.
- Add a dated Recent Model Retirements section mapping each retired
model to its replacement.
- Update AI docs examples and references in agent-settings.md,
inline-assistant.md, agent-panel.md, and llm-providers.md to use current
model names.
- Remove stale OpenAI model references in llm-providers.md that no
longer align with currently offered hosted models.

## Validation
- ./script/prettier
- ./script/check-todos

## Suggested .rules additions
- N/A

Release Notes:

- N/A

.rules

@@ -145,9 +145,17 @@ When an agent opens or updates a pull request, it must:
 - Avoid conventional commit prefixes in PR titles (`fix:`, `feat:`, `docs:`, etc.).
 - Avoid trailing punctuation in PR titles.
 - Optionally prefix the title with a crate name when one crate is the clear scope (for example, `git_ui: Add history view`).
-- Include a `Release Notes:` section in the PR body with one bullet:
+- Include a `Release Notes:` section as the final section in the PR body.
+- Use one bullet under `Release Notes:`:
   - `- Added ...`, `- Fixed ...`, or `- Improved ...` for user-facing changes, or
-  - `- N/A` for non-user-facing changes.
+  - `- N/A` for docs-only and other non-user-facing changes.
+- Format release notes exactly with a blank line after the heading, for example:
+
+```
+Release Notes:
+
+- N/A
+```
 
 # Crash Investigation