feat: support proxy environment variables for OAuth requests

[florianilch]

The OAuth service currently uses a custom http.Transport to configure TLS.

This change configures that transport to use also http.ProxyFromEnvironment to enable HTTP_PROXY, HTTPS_PROXY and NO_PROXY for token exchange and userinfo requests. The existing TLS configuration and other transport behavior remain unchanged.

Closes #998

Verification

make webui test and manual test

Summary by CodeRabbit

• Bug Fixes
  • OAuth network requests now respect proxy settings provided through the environment.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 4b42fe11-e3cc-4855-83f9-02e51227fc76

📥 Commits

Reviewing files that changed from the base of the PR and between b62bb2d and 97c8328.

📒 Files selected for processing (1)

• internal/service/oauth_service.go

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📝 Walkthrough

Walkthrough

The OAuth service configures its internal HTTP transport with http.ProxyFromEnvironment, enabling environment-based proxy handling for outbound OAuth requests.

Changes

OAuth proxy support

Layer / File(s)	Summary
Configure OAuth transport internal/service/oauth_service.go	NewOAuthService configures the OAuth HTTP transport to use environment-based proxy resolution.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Suggested reviewers: steveiliop56, dredsen, rycochet

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely matches the main change: enabling proxy env support for OAuth requests.
Linked Issues check	✅ Passed	The change enables OAuth requests to use ProxyFromEnvironment, matching the linked feature request for HTTP_PROXY/HTTPS_PROXY/NO_PROXY support.
Out of Scope Changes check	✅ Passed	The diff is narrowly scoped to OAuth transport proxy configuration and introduces no unrelated changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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