Preserve provider header overrides and apply custom_headers on Anthropic providers

[chindris-mihai-alexandru]

Summary
Forge currently applies global HTTP defaults in a way that can override provider-specific headers, and the Anthropic provider path does not forward provider.custom_headers at all.

Why this matters
Some third-party gateways accept requests only when a provider-specific header shape is preserved. The concrete reproducer on my side was a strict third-party gateway, but the bug is generic:

1. ForgeHttpInfra overwrites provider-specific X-Title and HTTP-Referer values with Forge defaults.
2. Anthropic provider requests do not append provider.custom_headers.
3. As a result, providers that require specific User-Agent / X-Title / HTTP-Referer / Origin headers cannot be integrated cleanly through provider.json alone.

Current behavior

• Provider-level User-Agent can survive, but X-Title and HTTP-Referer are overwritten by Forge defaults.
• Anthropic requests ignore provider.custom_headers entirely.

Expected behavior

• Forge default headers should only be applied when the provider did not already set them.
• Anthropic requests should honor provider.custom_headers the same way OpenAI requests do.

Suggested fix

• In crates/forge_infra/src/http.rs:
  • preserve provider overrides for User-Agent, X-Title, and HTTP-Referer
  • still add x-app-version and Connection defaults
• In crates/forge_repo/src/provider/anthropic.rs:
  • append provider.custom_headers to the outgoing request headers

Validation
The local patch was verified with:

• cargo test -q -p forge_repo -p forge_infra
• cargo check -q

And I also verified that the patch applies cleanly on upstream/main and the targeted test suite still passes after applying it.

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