fix(claude): add interleaved-thinking beta header, AMP gzip error decoding, normalizeClaudeBudget max_tokens

1. Always include interleaved-thinking-2025-05-14 beta header so that
   thinking blocks are returned correctly for all Claude models.

2. Remove status-code guard in AMP reverse proxy ModifyResponse so that
   error responses (4xx/5xx) with hidden gzip encoding are decoded
   properly — prevents garbled error messages reaching the client.

3. In normalizeClaudeBudget, when the adjusted budget falls below the
   model minimum, set max_tokens = budgetTokens+1 instead of leaving
   the request unchanged (which causes a 400 from the API).

internal/api/modules/amp/proxy.go

@@ -108,11 +108,6 @@ func createReverseProxy(upstreamURL string, secretSource SecretSource) (*httputi
 	// Modify incoming responses to handle gzip without Content-Encoding
 	// This addresses the same issue as inline handler gzip handling, but at the proxy level
 	proxy.ModifyResponse = func(resp *http.Response) error {
-		// Only process successful responses
-		if resp.StatusCode < 200 || resp.StatusCode >= 300 {
-			return nil
-		}
-
 		// Skip if already marked as gzip (Content-Encoding set)
 		if resp.Header.Get("Content-Encoding") != "" {
 			return nil