fix(pixel-settings): allow explicit thinking off when provider omits off from levels

[Vishaaallll]

Why this matters

In ods/bin/pixel_settings/contract.py, preview_preferences(preferences, capabilities) rejected thinking values that were not explicitly included in capabilities["supportedThinkingLevels"]. When an inference provider only lists active levels (such as ["low", "medium", "high"]) or does not support extended thinking at all ([]), an owner's explicit request to turn thinking off (thinking: "off") was rejected with SettingsError("thinking-level-not-supported").

This surgical fix permits thinking: "off" unconditionally, as disabling reasoning is the standard baseline state and does not require extended thinking capabilities.

Validation

• Baseline reproduction: verified that preview_preferences({"thinking": "off"}, caps) raised SettingsError when supportedThinkingLevels did not contain "off".
• Post-fix behavior: permits setting thinking: "off" while continuing to reject unsupported active levels.
• Telemetry: pixel-settings suite: 2 passed. New-test syntax and diff checks pass; new regression wired into Linux CI.

Overlap check

Inspected open/closed PRs. PR #5616 touched capabilities boundaries; PR #5696 touched provider limits. Neither addresses explicit thinking: "off" permission in preview_preferences.

Risk / AI disclosure

AI-assisted investigation, patch, and regression tests. Strictly relaxes rejection for the disabled thinking state. Independent review remains a gate.

Follow-up integration evidence

Composed locally with public-beta at 4fa6ad170 without conflicts. Regression tests pass.