fix(provider): preserve custom header overrides

[chindris-mihai-alexandru]

Summary

• preserve provider header overrides for User-Agent, X-Title, and HTTP-Referer
• forward provider.custom_headers on Anthropic requests

Why

Forge currently applies global HTTP defaults in a way that can override provider-specific headers, and the Anthropic provider path ignores provider.custom_headers entirely.

That makes some provider integrations impossible to express cleanly through provider config alone.

Changes

crates/forge_infra/src/http.rs

• only apply Forge defaults for User-Agent, X-Title, and HTTP-Referer when the provider did not already set them
• keep x-app-version and Connection defaults unchanged
• add a regression test for preserved provider overrides

crates/forge_repo/src/provider/anthropic.rs

• append provider.custom_headers to Anthropic request headers
• add a regression test covering provider-level custom headers on the Anthropic path

Verification

• cargo test -q -p forge_repo -p forge_infra
• cargo check -q
• patch also applies cleanly on upstream/main

Closes #3125

Co-Authored-By: ForgeCode noreply@forgecode.dev

[Copilot]

Pull request overview

Preserves provider-specific header overrides for key OpenRouter-related headers and ensures Anthropic requests forward provider-level custom headers, addressing integration limitations described in #3125.

Changes:

• Update Forge HTTP default-header application to avoid overwriting provider-supplied User-Agent, X-Title, and HTTP-Referer.
• Forward provider.custom_headers on the Anthropic provider path.
• Add regression tests for both behaviors.

Reviewed changes

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

File	Description
crates/forge_infra/src/http.rs	Applies Forge default headers conditionally (only when not already present) and adds a regression test.
crates/forge_repo/src/provider/anthropic.rs	Appends provider.custom_headers to Anthropic request headers and adds a regression test.

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[Copilot]

Provider custom headers are appended as raw strings, but later these headers are converted via create_headers(...), which currently expects valid header names/values and will panic at runtime if a provider config contains an invalid header. To avoid a crash from user-supplied config, validate header names/values here (e.g., HeaderName::try_from / HeaderValue::from_str) and either skip invalid entries with a warning or surface a structured error instead of panicking.

[chindris-mihai-alexandru]

Fixed in e56cd82. I now validate provider custom header names and values before appending them on the Anthropic path, skip invalid entries with a warning, and added a regression test to cover invalid custom headers.

[Copilot]

There is an unnecessary trailing semicolon after the if !headers.contains_key("HTTP-Referer") { ... } block. It’s harmless but tends to trigger lint warnings and is inconsistent with the other if blocks in this function; remove the semicolon for consistency.

Suggested change

	};
	}

[chindris-mihai-alexandru]

Fixed in e56cd82. I removed the trailing semicolon after the HTTP-Referer override block for consistency with the surrounding conditionals.