fix(retry): guard RetryBudget mutable config against data race

UpdateConfig writes tokensPerSecond/maxTokens/enabled as plain fields while
the refill goroutine and AllowRetry readers touch them concurrently. Guard
all reads/writes of the mutable config with an RWMutex.

Adds TestRetryBudget_ConcurrentUpdateConfig, which races UpdateConfig
against AllowRetry/GetStats and is verified clean under -race.

Also drop the package-level min() that shadowed the Go 1.25 builtin.

request_coalescing.go

@@ -234,10 +234,3 @@ func InitializeRequestCoalescer(enabled bool, logger *libpack_logger.Logger, mon
 func GetRequestCoalescer() *RequestCoalescer {
 	return requestCoalescer
 }
-
-func min(a, b int) int {
-	if a < b {
-		return a
-	}
-	return b
-}