feat: support reasoning in responses chat adapter

[Rader]

Summary:

• Extends the chat adapter to correctly parse and transform reasoning outputs (e.g., reasoning_content, reasoning) into standard Responses items while preserving normal text, ensuring compatibility across upstreams like DeepSeek and GLM.
• Introduces a configurable metadata field for upstreams to define custom reasoning request mappings, allowing the gateway to translate standard reasoning.effort parameters into provider-specific chat body fields without hardcoding logic.
• Enhances input validation and token usage tracking to handle reasoning summaries, drop unsupported tools gracefully, and ensure accurate token counting even when upstream usage data is missing.

[Rader]

Depends on #1240

[HaiHui886]

lgtm