fix(config): support float64 env values in getDetailsFromEnv

The generic typed env reader only handled string/int/bool; any float64 key
fell through to default, so CIRCUIT_FAILURE_RATIO, CIRCUIT_BACKOFF_
MULTIPLIER and RETRY_BUDGET_TOKENS_PER_SEC were silently impossible to
override via environment (always the compile-time default).

Add a float64 case mirroring the int one: parse the GMP_ prefixed key,
then the plain key, else the default (envutil has no GetFloat).

Test: extended Test_getDetailsFromEnv with float-valid and float-malformed
cases.

main_test.go

@@ -154,6 +154,8 @@ func (suite *Tests) Test_getDetailsFromEnv() {
 		{"default", "envValue", "string value", "TEST_STRING", "envValue"},
 		{0, 123, "int value", "TEST_INT", "123"},
 		{false, true, "bool value", "TEST_BOOL", "true"},
+		{0.5, 0.25, "float value", "TEST_FLOAT", "0.25"},
+		{0.5, 0.5, "float default on malformed value", "TEST_FLOAT_BAD", "not-a-number"},
 		{"default", "default", "default value", "NON_EXISTENT", ""},
 	}