fix forced tool_choice 400 on claude 5.5 models and surface openrouter upstream errors

[RaajeevChandran]

Summary

Resolves #3051

• Delegated tasks to anthropic/claude-sonnet-5.5 failed with HTTP 400 before any output because Osaurus sent tool_choice: "required", which Opus 5.5, Sonnet 5.5, Fable 5.1 and Mythos 5.1 reject on every host
• RemoteReasoningPolicy.sanitizedToolChoice now downgrades required and named tool choices to auto for those models, matched on the bare model id so OpenRouter (anthropic/claude-sonnet-5.5) and native (claude-sonnet-5-5) ids are both covered
• The check runs before the provider switch, so OpenRouter, native Anthropic and other hosts all get it. Haiku 5.5 and older Claude models keep forced tool use
• OpenRouter's generic "Provider returned error" now includes the upstream reason from metadata.raw, so the failure explains itself
• Numeric error codes (OpenRouter sends 400 as a number) are now shown alongside the message
• Added tests for the downgrade across hosts and model id forms, the pass-through for older models, and both JSON and plain-text metadata.raw

Changes

• Behavior change
• UI change (screenshots below)
• Refactor / chore
• Tests
• Docs

Checklist

• I have read CONTRIBUTING.md
• I added/updated tests where reasonable
• I updated docs/README as needed
• I verified build on macOS with Xcode 16.4+