fix(runtime): classify provider capacity errors

[CxHsin]

Summary

Fixes #3341

Provider resource-exhausted failures now retain a stable provider_capacity classification, use bounded retry metadata, and receive capacity-specific Desktop error and recovery guidance instead of Unknown error and direct-retry advice.

Verification

• npm install succeeded and installed @ai-sdk/code-mode@1.0.23.
• npm run build passed for all workspaces.
• Runtime provider classification tests passed: 11/11.
• Desktop provider capacity presentation tests passed: 2/2.
• Full npm test was attempted; it remains blocked by unrelated Windows environment failures including symlink permissions (EPERM), SQLite locks (EBUSY), missing Rive CLI, and workspace timeouts.

AI use

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Codex implemented the runtime classification, retry mapping, Desktop copy/recovery behavior, and regression tests. A Generated-by: Codex trailer is present in the commit.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — provider capacity failures now have stable classification and wait/switch-model guidance.
• No