fix(networking): honor HTTP(S)_PROXY in HttpClientBuilder.Build()

[schonmann]

Problem

• HttpClientBuilder.Build() builds its http.Transport without a Proxy field.
• That means clients it produces ignore HTTP_PROXY / HTTPS_PROXY / NO_PROXY (a zero-value http.Transport has Proxy: nil, unlike http.DefaultTransport).
• The embedded auth server's upstream OAuth/DCR client is built this way (pkg/authserver/upstream/oauth2.go → newHTTPClientForHost → NewHostScopedClientBuilder().Build()).
• Where the upstream is only reachable via an explicit forward proxy, the token-exchange and DCR calls bypass the proxy and fail even with the proxy env vars set.
• The vMCP tool-forwarding path is unaffected, since it uses CloneDefaultTransportWithDialControl, which already sets Proxy: http.ProxyFromEnvironment.

flowchart LR
    Pod["MCPRemoteProxy pod<br/>(HTTPS_PROXY set)"]

    Pod -->|"tool-forwarding client<br/>CloneDefaultTransportWithDialControl"| P1["honors proxy"]
    Pod -->|"auth-server OAuth client<br/>Build → Proxy: nil"| P2["ignores proxy"]

    P1 --> FP["forward proxy"] --> UP1["upstream MCP host<br/>reachable"]
    P2 --> DIRECT["direct egress"] --> UP2["IP-allowlisted token<br/>endpoint → 403"]

    classDef ok fill:#e7f7ec,stroke:#2e7d32,color:#1b3d24;
    classDef bad fill:#fdecea,stroke:#c62828,color:#5b1a15;
    class P1,FP,UP1 ok;
    class P2,DIRECT,UP2 bad;

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Fixes #6671

Fix

• Set Proxy: http.ProxyFromEnvironment on the transport in Build(), matching http.DefaultTransport and the vMCP path.
• Adds a regression test asserting the built transport sets Proxy.

Note

• When the proxy address is itself private, the private-IP dial guard blocks the proxy connection.
• That case is covered by the existing AllowPrivateIPs option.

Type of change

• Bug fix

Test plan

• Unit tests (task test) — added TestHttpClientBuilder_BuildSetsProxyFromEnvironment

Note: the suite was not run on the authoring machine (no Go toolchain available there); relying on CI to execute task test / task lint-fix.

API Compatibility

• This PR does not break the v1beta1 API (it only touches pkg/networking).

Does this introduce a user-facing change?

Yes — HTTP clients built via pkg/networking (including the auth server's upstream OAuth/DCR calls) now honor HTTP(S)_PROXY / NO_PROXY, so deployments behind an explicit forward proxy work as expected.

[Copilot]

Pull request overview

This PR fixes HttpClientBuilder.Build() so HTTP clients created via pkg/networking honor HTTP_PROXY / HTTPS_PROXY / NO_PROXY, aligning behavior with http.DefaultTransport and the existing vMCP transport path; this unblocks proxy-only upstream OAuth/DCR calls from the embedded auth server.

Changes:

• Set http.Transport.Proxy = http.ProxyFromEnvironment in HttpClientBuilder.Build().
• Add a regression test asserting the built transport has a non-nil Proxy function.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pkg/networking/http_client.go	Ensures builder-created transports use proxy env vars by wiring ProxyFromEnvironment.
pkg/networking/http_client_test.go	Adds a regression test to prevent dropping the proxy behavior in future refactors.

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[schonmann]

Hi team, quick pull request for your consideration! 👋

In essence -> this will honor the HTTP(S)_PROXY env, making token endpoints from MCPs behind firewall and with a strict source-IP allowlist (i.e. including proxy's IP range) functional.

Thx!

[jhrozek]

Thanks for the patch. Do you think it's safe to always pass on the environment proxy variables?
What about adding a new option along the lines of the existing WithPrivateIPs something like WithProxyFromEnvironment that would enable the proxy environment values only when set?

[schonmann]

Hey, thanks for checking @jhrozek!

Dug into this a bit, and I think always passing is actually a safe choice here. Why:

• From what I can tell, Build() was the only spot in pkg/networking that wasn’t honoring the proxy. CloneDefaultTransportWithDialControl and Go’s own http.DefaultTransport already set Proxy: ProxyFromEnvironment.
• The "enable only when set" behavior kind of comes for free: ProxyFromEnvironment returns a nil URL when no HTTP(S)_PROXY is exported, and the net/http docs say "if Proxy is nil or returns a nil *URL, no proxy is used".
• With no env var it behaves exactly like today, and NO_PROXY handles the opt out.

That's why I'd lean away from a separate WithProxyFromEnvironment flag in this case, specifically. Thoughts?