feat(runtime): add model context and session thinking controls

[GTC2333]

Summary

Stage 3 PR 8/9.

• configure provider context-window presets through backend and Web settings
• propagate context-window metadata into provider sessions and compaction policy
• add a session-wide thinking level shared by principal and subagents
• allow the thinking level to be changed from the composer
• persist and expose the effective session control state

Verification

• root typecheck
• 163 targeted runtime/backend tests
• 59 targeted Web tests
• Web production build
• diff whitespace validation

Merge notes

Targets main directly and is independent of the plugin PRs.

[GTC2333]

Addressed both code-quality comments in 7b7ce89 by removing the unused useCallback and SessionMessageEntry imports. Root typecheck, 59 targeted Web tests, and the Web production build pass.

[Fxde42]

Request changes：已有 session 的 Thinking 控件仍由当前全局 activeProvider/selectedModel 判定，而不是该 session 实际绑定的模型。切换全局 provider 后，旧 session 可能显示禁用的 off，或错误开放 Thinking。建议把服务端计算的 reasoningSupported 作为 Session contract 的权威字段；已有 session 读取该字段，只有 draft 使用当前全局 provider。请补跨 provider 切换的回归测试。

[GTC2333]

Addressed the requested Session authority change in 44a2147. The runtime now computes, persists, restores, and exposes reasoningSupported for the provider/model bound to the session. Existing sessions use that field exclusively; only drafts consult the current global provider. Added regression coverage for switching the global provider in both directions and for draft behavior. Validation: root typecheck, 86 runtime tests, 62 Web tests, and Web production build.