fix: add xAI/Grok API compatibility

[golah]

Summary

ForgeCode currently fails to connect to xAI's API (Grok models) with "Client specified an invalid argument" error on every request.

Root Cause

xAI's API rejects null values in enum arrays within tool schemas. The fs_search tool's output_mode parameter generates "enum": ["content", "files_with_matches", "count", null] via schemars, which xAI rejects.

Additionally, xAI doesn't support several parameters that MakeOpenAiCompat passes through (stream_options, max_completion_tokens, reasoning_effort, etc.).

Fix

1. Enable EnforceStrictToolSchema for xAI — this existing transformer converts nullable enums to anyOf format that xAI accepts.

2. Add MakeXaiCompat transformer — handles xAI-specific parameter differences:

   • Removes stream_options (not reliably supported)
   • Reverts max_completion_tokens back to max_tokens (xAI uses max_tokens)
   • Strips reasoning_effort, thinking, logit_bias, top_logprobs, seed, prediction, parallel_tool_calls

Testing

Tested locally with:

• grok-4-1-fast-reasoning — tool calling, file reading, code analysis ✅
• grok-4.20-0309-reasoning — code editing with verification ✅
• grok-4.20-0309-non-reasoning — basic chat and edits ✅

Files Changed

• crates/forge_app/src/dto/openai/transformers/make_xai_compat.rs (new) — xAI parameter compatibility transformer
• crates/forge_app/src/dto/openai/transformers/pipeline.rs — add xAI to strict schema condition + wire MakeXaiCompat into pipeline
• crates/forge_app/src/dto/openai/transformers/mod.rs — register new module

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