fix(plugins): preserve declared thinkingLevels on plugin providers

[ZxlDragonDoctor]

Problem

Plugin-declared providers (contributes.providers) reach the model picker with an empty thinking-level menu even when the manifest declares thinkingLevels / defaultThinkingLevel. The same models added by hand in Settings keep their levels.

Root cause in crates/host-core/src/plugins/providers.rs declared_providers(): ModelBinding is built with thinking_levels: Vec::new() and default_thinking_level: None — the manifest fields are never read. packages/plugin-sdk/src/index.ts also omitted the fields from PluginProviderModelContrib.

Solution

• Read thinkingLevels / defaultThinkingLevel through normalize_thinking_levels (same helper the runtime already uses).
• Drop a default that names a level the model does not offer.
• Document both fields on PluginProviderModelContrib.
• Purely additive: manifests without the fields keep today's empty binding.

Addresses the reporter analysis and attached patch on #777 (no separate maintainer design note on the issue).

Testing

• Offline contract tests on this host (cargo test blocked: MSVC link.exe not installed):
  • Command: py -3.13 offline_test_plugin_thinking_levels.py (work tree)
  • Result: offline_plugin_thinking_levels: 5 passed
  • Cases: declared levels round-trip; canonicalisation/dedupe; default outside list dropped; malformed list ignored; patch surfaces present in providers.rs / SDK / tests.
• In-tree Rust unit tests for CI: cargo test -p host-core plugins::providers
  • declared_thinking_levels_survive_into_the_row
  • declared_thinking_levels_are_normalized
  • a_default_outside_the_declared_list_is_dropped
  • a_malformed_thinking_levels_field_is_ignored

Agent dimension

tool-retrieval (plugin-declared provider/model catalog reaches the model picker)

Core value

Plugin-contributed models keep their declared reasoning levels instead of a silently empty menu.

Issue alignment

• Addresses reporter root-cause analysis and proposed fix on [Bug] When using a plugin to connect to the AI model service, the inference level cannot be adjusted. #777.
• No maintainer design note contradicting this direction; comment on [Bug] When using a plugin to connect to the AI model service, the inference level cannot be adjusted. #777 noted plugin/native incompatibility generally, not a won't-do on thinkingLevels.

Fixes #777

[vastsa]

Verified and merged. The reported bug was real: before this change, declared_providers() discarded plugin manifest thinking-level fields by constructing every ModelBinding with an empty thinking_levels and no default.

PR #796 preserves thinkingLevels / defaultThinkingLevel, normalizes canonical values, rejects an explicit default outside the declared set, and adds Rust coverage plus SDK types. I added only the landing blockers on top of the contributor commit: formatter output and a non-rewriting merge of the latest main.

Landed as merge commit 991b1b132. Base check, Rust format/lint/test, and JS build/typecheck/lint/test all passed. A separate post-merge documentation follow-up records the manifest contract and E2E expectation.