fix(openai): relax tool call validation for Ollama compatibility

[Gustave-241021]

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related link:

issue: charmbracelet/crush#1767
crush's fix: charmbracelet/crush#1774

Problem Attribution:

Ollama's OpenAI-compatible streaming API does not include id and type fields in tool call delta chunks, unlike the standard OpenAI format. This causes the fantasy library's strict validation to fail, resulting in Crush getting stuck in a "waiting" state.

Fix: Relaxed tool call validation in language_model.go to handle missing fields gracefully:

• Default type to "function" when not provided
• Generate synthetic ID tool-call-{index} when ID is missing
• Skip empty delta chunks instead of erroring
• Ensure consistent ID usage throughout the streaming handler

Risk Mitigation:

• Only applies defaults when fields are missing, preserving standard OpenAI behavior
• All 7 fantasy packages pass existing tests, confirming no regression
• Explicit type validation still catches wrong types (e.g., type: "invalid")