test(mimo): add completePrompt and advanced streaming tests for Mimo provider

[proyectoauraorg]

Summary

This PR adds 12 new test cases for the Mimo provider, increasing test coverage from 33 to 45 tests.

New Tests Added

completePrompt tests (8)

1. Successful response with proper message content
2. Empty prompt handling
3. maxTokens parameter passed to API
4. Thinking/reasoning content in response
5. reasoningEffort options (low/medium/high)
6. API error handling with graceful fallback
7. Empty choices response handling
8. Special characters and Unicode in prompts

Advanced streaming tests (4)

1. Malformed JSON chunk resilience
2. Error events mid-stream handling
3. Abort signal cancellation
4. Prompt caching support (cache_control)

Testing

All 45 tests pass with zero warnings:

• Pre-existing tests: 33
• New tests: 12
• Total: 45/45 passing

Motivation

These tests cover critical untested paths in the Mimo provider:

• Error handling: API errors, malformed responses, empty choices
• Edge cases: empty prompts, special characters, Unicode
• Streaming resilience: malformed chunks, error events, cancellation
• Advanced features: prompt caching, reasoning effort options

This improves reliability and makes future refactoring safer.