PR #51946 broke `Model::Custom` thinking behavior: `mode()`,
`supports_thinking()`, and `supports_adaptive_thinking()` all inferred
capabilities from hardcoded built-in model lists, so any `Custom`
variant always fell back to `Default` regardless of its configured
`mode` field.

### Fixes

- **`Model::mode()`** — `Custom` now short-circuits to `mode.clone()`
before the built-in inference logic
- **`Model::supports_thinking()`** — `Custom` returns `true` when `mode`
is `Thinking { .. }` or `AdaptiveThinking`
- **`Model::supports_adaptive_thinking()`** — `Custom` returns `true`
when `mode` is `AdaptiveThinking`

Built-in model behavior is unchanged.

### Tests

Three regression tests added covering the three `Custom` mode cases:
explicit `Thinking`, `AdaptiveThinking`, and `Default` (which must
disable both flags).

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed custom Anthropic models losing their configured
thinking/adaptive-thinking mode after the thinking-toggle refactor
(#51946)

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>