Implements unified model routing

Migrates the AMP module to a new unified routing system, replacing the fallback handler with a router-based approach.

This change introduces a `ModelRoutingWrapper` that handles model extraction, routing decisions, and proxying based on provider availability and model mappings.
It provides a more flexible and maintainable routing mechanism by centralizing routing logic.

The changes include:
- Introducing new `routing` package with core routing logic.
- Creating characterization tests to capture existing behavior.
- Implementing model extraction and rewriting.
- Updating AMP module routes to utilize the new routing wrapper.
- Deprecating `FallbackHandler` in favor of the new `ModelRoutingWrapper`.

internal/api/modules/amp/model_mapping.go

@@ -276,6 +276,22 @@ func (m *DefaultModelMapper) GetMappings() map[string]string {
 	return result
 }
 
+// GetMappingsAsConfig returns the current model mappings as config.AmpModelMapping slice.
+// Safe for concurrent use.
+func (m *DefaultModelMapper) GetMappingsAsConfig() []config.AmpModelMapping {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+
+	result := make([]config.AmpModelMapping, 0, len(m.mappings))
+	for from, to := range m.mappings {
+		result = append(result, config.AmpModelMapping{
+			From: from,
+			To:   to,
+		})
+	}
+	return result
+}
+
 type regexMapping struct {
 	re *regexp.Regexp
 	to string