fix(kosong): sanitize surrogates before Kimi requests

[he-yufeng]

Summary

• sanitize lone UTF-16 surrogate code units before sending Kimi chat-completion requests
• cover system prompts and converted message payloads, including history text and tool-call arguments
• add a regression test that JSON-encodes the outgoing request with ensure_ascii=False

Fixes #2320

To verify

• uv run pytest packages/kosong/tests/test_chat_provider.py -q -p no:cacheprovider
• uv run --package kosong --group dev pytest packages/kosong/tests -q -p no:cacheprovider
• uv run --package kosong --group dev ruff check packages\kosong\src\kosong\chat_provider\kimi.py packages\kosong\tests\test_chat_provider.py
• uv run --package kosong --group dev pyright packages\kosong\src\kosong\chat_provider\kimi.py packages\kosong\tests\test_chat_provider.py
• git diff --check

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.