Fix ProxyOriginOf for opaque/scheme-less URLs and userinfo leakage

[yrobla]

Context

Identified in PR #5142 review by @JAORMX and Copilot.

Problem

ProxyOriginOf in pkg/llmgateway/config.go has several correctness gaps:

1. Opaque URLs: url.Parse("localhost:14000/v1") parses as Scheme=localhost, Opaque=14000/v1. The function clears Path but not Opaque, so the path survives in the result — contradicting the doc comment that says "only the scheme and host remain."

2. ForceQuery: Clearing RawQuery alone may still produce a trailing ? if u.ForceQuery is true (e.g. parsing http://host/path?). u.ForceQuery must also be set to false.

3. Fragment: Fragment is not cleared — if ProxyBaseURL ever contains one, it leaks into the written settings file.

4. Userinfo: u.User is not cleared. If ProxyBaseURL ever carries user credentials, they'd be persisted to a user-readable settings file. Should be set to nil.

5. Validation: There's no guard that u.Host != "" after parsing. For scheme-less inputs the function may return a non-origin result silently.

Suggested fix

func ProxyOriginOf(rawURL string) string {
    u, err := url.Parse(rawURL)
    if err != nil || u.Host == "" {
        return rawURL
    }
    return (&url.URL{Scheme: u.Scheme, Host: u.Host}).String()
}

Explicitly reconstructing from Scheme+Host is simpler and correct by construction — no fields to forget to clear.

Test cases to add (pkg/llmgateway/config_test.go)

• Empty input
• Opaque URL (localhost:14000/v1)
• Scheme-less host:port
• URL with userinfo
• IPv6 host
• Fragment-only (http://host/path#frag)
• ForceQuery (http://host/path?)
• Valid full URL (happy path)

References

• PR Correct Gemini CLI LLM gateway config to proxy mode #5142 review comment by @JAORMX
• PR Correct Gemini CLI LLM gateway config to proxy mode #5142 review comments by Copilot (Fragment and ForceQuery)

[yrobla]

This was already fixed in #5142 — the review feedback was addressed before merge. was introduced with the correct reconstruction-from- pattern, and all test cases from the issue description are present and passing.