fix: use vscode.env.asExternalUri() for Claude Code OAuth in remote environments

[roomote-v0]

This PR attempts to address Issue #10531. Feedback and guidance are welcome.

Problem

The OAuth flow uses a hardcoded localhost redirect URI (http://localhost:54545/callback) which breaks authentication in remote environments like GitHub Codespaces because the browser cannot reach localhost.

Solution

This PR implements the recommended vscode.env.asExternalUri() API solution to dynamically convert the localhost redirect URI to an externally accessible URL in remote environments.

How it works

• In local VS Code: asExternalUri() returns the original URI unchanged
• In GitHub Codespaces: Returns a forwarded URL like https://<codespace-name>-54545.app.github.dev/callback
• In other remote environments: Returns the appropriate externally-accessible URL

Changes

1. Modified startAuthorizationFlow() to:

   • Accept vscode.env and vscode.Uri.parse as parameters
   • Use asExternalUri() to transform the localhost callback URL
   • Store the resolved redirect URI for use in token exchange

2. Updated buildAuthorizationUrl() to accept an optional custom redirectUri parameter

3. Updated exchangeCodeForTokens() to accept an optional custom redirectUri parameter (must match the one used in authorization)

4. Added tests for custom redirect URI functionality

This approach is backwards compatible - local users will not notice any change, while remote users will have a working OAuth flow.

Fixes #10531

---

Important

The PR updates OAuth flow to use vscode.env.asExternalUri() for remote environments, ensuring compatibility with platforms like GitHub Codespaces.

• Behavior:
  • startAuthorizationFlow() in ClaudeCodeOAuthManager now uses vscode.env.asExternalUri() to convert localhost URIs for remote environments.
  • Supports GitHub Codespaces by converting to URLs like https://<codespace-name>-54545.app.github.dev/callback.
  • Backward compatible for local VS Code users.
• Functions:
  • startAuthorizationFlow() accepts vscode.env and vscode.Uri.parse as parameters.
  • buildAuthorizationUrl() and exchangeCodeForTokens() updated to accept optional redirectUri.
• Tests:
  • Added tests in oauth.spec.ts for custom redirect URI handling.

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[roomote-v0]

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Review complete. No issues found.

The implementation correctly uses vscode.env.asExternalUri() to support remote environments (GitHub Codespaces, etc.) for Claude Code OAuth authentication.

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