feat(ai): add failover rule helpers

[goravel-coder]

Summary

• Adds AI failover rule helpers that external providers can use to classify provider-specific errors.
• Supports plain substring patterns and slash-delimited regex patterns for custom failover reasons.
• Removes the embedded OpenAI provider now that OpenAI lives in goravel/openai.

Why

The framework should stay provider-agnostic: it should react to FailoverError, while provider packages decide how their own errors map to failover reasons. The new helper API lets external providers compile and apply user-provided failover rules without moving provider-specific matching into the framework runtime.

package config

import (
	openai "github.com/goravel/openai"

	"github.com/goravel/framework/facades"
)

func init() {
	config := facades.Config()
	config.Add("ai", map[string]any{
		"default": "openai",
		"providers": map[string]any{
			"openai": map[string]any{
				"via": openai.New(),
				"failover": map[string][]string{
					"context_length_exceeded": {
						"maximum context length",
						"/(?i)context.*length/",
					},
				},
			},
		},
	})
}

This keeps OpenAI-specific behavior in goravel/openai while the framework owns only the shared contract and helper functions. Providers can call the helpers when they build or execute requests, then return NewFailoverError through the normal failover path.

[codecov]

Codecov Report

❌ Patch coverage is 70.58824% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.28%. Comparing base (9e0d1e9) to head (c5029bf).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
ai/failover.go	70.58%	10 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1484      +/-   ##
==========================================
- Coverage   69.40%   69.28%   -0.13%     
==========================================
  Files         377      377              
  Lines       30190    29613     -577     
==========================================
- Hits        20954    20517     -437     
+ Misses       8256     8151     -105     
+ Partials      980      945      -35     

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[hwbrzzl]

Use t.Run style for these cases.