oss fs for c#/kotlin

[ninan-nn]

Summary

Add OSSFS volume backend support to the C# and Kotlin sandbox SDKs, and align the JavaScript SDK docs/comments now that OSSFS support has landed there.

Closes#476.

Changes:

• sdks/sandbox/csharp: add OSSFS model, expose Volume.ossfs, and add serialization coverage for sandbox create payloads
• sdks/sandbox/kotlin: add domain OSSFS model/builder, extend Volume validation to host/pvc/ossfs, wire OSSFS into API conversion, and add model + adapter tests
• sdks/sandbox/javascript: update README/README_zh and public option comments so docs match the current ossfs support

Testing

• Unit tests
  • Added/updated C# model serialization tests
  • Added/updated Kotlin model and adapter serialization tests
• Integration tests
• e2e / manual verification

Breaking Changes

• None
• Yes (describe impact and migration path)

Checklist

• Linked Issue or clearly described motivation
• Added/updated docs (if needed)
• Added/updated tests (if needed)
• Security impact considered
• Backward compatibility considered

[hittyt]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep it up!

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[hittyt]

LGTM