fix: rename gateway auth header from X-AnyLLM-Key to AnyLLM-Key

[tbille]

Description

Renames the gateway authentication header from X-AnyLLM-Key to AnyLLM-Key to comply with RFC 6648 (no X- prefix), matching the rename in gateway PR #45.

Changes:

• Updated GATEWAY_HEADER_NAME constant in src/providers/gateway/mod.rs
• Updated associated doc comments referencing the header name
• Renamed test function and updated header matcher in tests/test_gateway.rs

No backward compatibility concern since the gateway is not yet live.

PR Type

• 🐛 Bug Fix

Relevant issues

Aligns with gateway PR #45 header rename.

Checklist

• I understand the code I am submitting.
• I have added unit tests that prove my fix/feature works
• I have run this code locally and verified it fixes the issue.
• New and existing tests pass locally (cargo test --all-features)
• Code passes linting (cargo clippy --all-features -- -D warnings)
• Code is formatted (cargo fmt)
• Documentation was updated where necessary
• I have read and followed the contribution guidelines
• AI Usage:
  • No AI was used.
  • AI was used for drafting/refactoring.
  • This is fully AI-generated.

AI Usage Information

• AI Model used: Claude claude-opus-4-6 (anthropic/claude-opus-4-6)
• AI Developer Tool used: OpenCode
• Any other info you'd like to share: Straightforward constant + doc comment rename with matching test updates.