Context

PR #1684 added enable_extended_context support. The tester found (GAP-1) that while the disabled path has a build_request() integration test verifying no beta header, the enabled path does not have an equivalent test verifying the HTTP-level header is actually present in the assembled request.

Task

Add a unit test in crates/zeph-llm/src/claude.rs that:

1. Constructs a ClaudeProvider with enable_extended_context = true
2. Calls build_request() (or the equivalent request assembly method)
3. Asserts that the anthropic-beta header value contains context-1m-2025-08-07

This complements the existing extended_context_enabled_has_beta_header test which tests beta_header() in isolation.

Follows up on #1649 (tester GAP-1).