Fix interceptor auth injection on Windows

[Netzhangheng]

Summary

• fix server auth interceptor injection to preserve the interceptor instance binding
• avoid calling the interceptor use method without its receiver
• unblock plugin initialization on Windows when auth injection reaches the interceptor path

Validation

• reproduced the his._fns failure from the packaged plugin on Windows
• applied the equivalent source fix in src/shared/opencode-server-auth.ts
• ran bun run typecheck
• ran bun run build
• verified locally that the packaged plugin no longer throws the interceptor binding error during initialization

[github-actions]

All contributors have signed the CLA. Thank you! ✅
_{Posted by the CLA Assistant Lite bot.}

[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: Fixes a standard JS context binding bug by ensuring the 'use' method is called with its receiver. This is a low-risk, 1-line fix for a reported runtime error.}

[Netzhangheng]

I have read the CLA Document and I hereby sign the CLA

[cubic-dev-ai]

0 issues found across 1 file (changes from recent commits).

_{Auto-approved: Fixes a binding error by correctly setting the 'this' context for the interceptor's 'use' method, resolving a reported runtime failure on Windows with minimal risk.}