fix: apply custom_headers to Anthropic requests and honor API context_length

[romiluz13]

Summary

• Bug 1: custom_headers not applied to Anthropic requests — Provider-level custom_headers defined in .forge.toml are parsed during config loading but never injected into outgoing HTTP requests for Anthropic-type providers. This breaks any proxy that requires alternative header names (e.g., Azure APIM expects api-key instead of x-api-key). Fixed by appending custom_headers in get_headers() after standard headers are built.

• Bug 2: API-reported context_length ignored — The Model struct in the Anthropic response DTO only deserializes id and display_name, discarding the context_length field returned by /v1/models. Instead, get_context_length() hardcodes all Claude models to 200K. This prevents custom providers from advertising larger context windows (e.g., 1M tokens with the context-1m-2025-08-07 beta header). Fixed by adding context_length: Option<u64> to Model and preferring the API value over the hardcoded fallback.

Motivation

When using Forge with Azure APIM-based proxies (which require api-key header instead of x-api-key), the [providers.custom_headers] config option has no effect for Anthropic providers. Users have to stand up a local reverse proxy just to translate headers — defeating the purpose of the config.

Similarly, operators serving a custom /v1/models endpoint that reports 1M context windows (enabled via the Anthropic beta header) cannot propagate that information to Forge because the context_length field is never deserialized.

Changes

File	Change
crates/forge_repo/src/provider/anthropic.rs	Append provider.custom_headers in get_headers() + test
crates/forge_app/src/dto/anthropic/response.rs	Add context_length field to Model, prefer API value over fallback + tests

Test plan

• New test: test_get_headers_includes_custom_headers — verifies custom headers are appended alongside standard headers
• New test: test_model_conversion_api_context_length_takes_precedence — verifies API-reported 1M overrides hardcoded 200K
• New test: test_model_conversion_unknown_model_with_api_context_length — verifies unknown models work with API context_length
• Existing tests updated with context_length: None to match new struct field
• CI: cargo test passes (no Rust toolchain available locally — relying on CI)

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