fix: make translated Anthropic requests provider-valid

[alongubkin]

Summary

• supply a bounded output-token default when OpenAI Chat Completions or Responses requests are translated to Anthropic Messages without a limit
• treat an explicit null OpenAI limit as unspecified while preserving explicit numeric limits exactly
• make the direct-Anthropic integration mock validate the provider-required max_tokens field and a real translated user message
• cover both OpenAI request protocols with focused translation tests

Fixes ALIEN-571.

Validation

• cargo fmt --all -- --check
• cargo test -p alien-ai-gateway (89 unit tests, 9 runnable integration/live tests passed; credentialed cloud tests remained ignored)
• cargo clippy -p alien-ai-gateway --all-targets --locked (passes with existing warnings)
• strict -D warnings is currently blocked by existing workspace and gateway warnings unrelated to this diff

[greptile-apps]

Greptile Summary

The PR makes OpenAI-to-Anthropic request translation provider-valid by supplying a bounded max_tokens default when the OpenAI limit is omitted or null.

• Preserves explicit numeric output-token limits across both OpenAI protocols.
• Adds focused unit and integration coverage for translated Anthropic requests.
• Reduces Rust CI build and test concurrency.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
crates/alien-ai-gateway/src/protocol.rs	Adds the required Anthropic output-token default and tests omitted, null, and explicit OpenAI limits.
crates/alien-ai-gateway/tests/integration.rs	Strengthens direct-Anthropic integration coverage by validating native and translated request bodies separately.
.github/workflows/ci-fast.yml	Reduces Rust compilation and nextest concurrency from twelve workers to eight.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
    A[OpenAI Chat Completions] --> T[Protocol translator]
    B[OpenAI Responses] --> T
    T --> L{Output limit numeric?}
    L -->|Yes| P[Preserve explicit limit]
    L -->|No: omitted or null| D[Use 4096-token default]
    P --> M[Anthropic Messages request]
    D --> M
    M --> U[Anthropic provider]

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_{Reviews (2): Last reviewed commit: "ci: reduce Rust test memory pressure" | Re-trigger Greptile}