Add Azure FW Kimi K3 support

[homanp]

Summary

• expose the Azure FW-Kimi-K3 deployment as fw-kimi-k3
• allow strict Chat Completions routes to drop unsupported per-message fields
• preserve reasoning_content while omitting Azure's rejected reasoning alias during tool loops

Test plan

• Run cargo test
• Run cargo clippy --all-targets
• Verify a streaming Claude Code multi-turn tool loop through Railway returns HTTP 200

[open-cla]

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