feat(thinking): add xAI provider support with reasoning.effort implementation

- Implemented `xAI` provider for thinking configurations with support for reasoning.effort levels.
- Registered `xAI` in available providers and updated relevant APIs for compatibility.
- Added unit tests for `xAI` provider functionality, including fallback logic for unsupported levels.
- Integrated `xAI` with executor handling and ensured conformance with OpenAI-compatible standards.

internal/thinking/provider/xai/apply.go

@@ -0,0 +1,26 @@
+// Package xai implements thinking configuration for xAI Grok Responses API models.
+//
+// xAI models use the OpenAI Responses API compatible reasoning.effort format
+// with discrete levels.
+package xai
+
+import (
+	"github.com/router-for-me/CLIProxyAPI/v7/internal/thinking"
+	"github.com/router-for-me/CLIProxyAPI/v7/internal/thinking/provider/codex"
+)
+
+// Applier implements thinking.ProviderApplier for xAI models.
+type Applier struct {
+	codex.Applier
+}
+
+var _ thinking.ProviderApplier = (*Applier)(nil)
+
+// NewApplier creates a new xAI thinking applier.
+func NewApplier() *Applier {
+	return &Applier{}
+}
+
+func init() {
+	thinking.RegisterProvider("xai", NewApplier())
+}