DynamicTokenBucket::consumeWithBorrowNonBlocking will return incorrect value with very low rate

The following unit test demonstrates the issue:

```
TEST(TokenBucketTest, CanConsumeFirstTokenLowRate) {
folly::DynamicTokenBucket tokenBucket;
ASSERT_EQ(tokenBucket.consumeWithBorrowNonBlocking(1.0 /** toConsume*/, .0001 /** rate*/, 1.0 /** burstSize*/), 0.0);
}
```

IIUC, this should succeed because the bucket is initialized with 1.0 token, and we are only attempting to consume this first token, and no wait should be required. However, for an excessively low rate (like .0001, this does not repro with .001), the test fails instead:

```
{"t":{"$date":"2025-05-02T12:53:32.953+00:00"},"s":"I", "c":"TEST", "id":8423378, "ctx":"main","msg":"Test Failed","attr":{"testName":"TokenBucketTest/CanConsumeFirstTokenLowRate","exception":"TestAssertionFailureException","error":"Expected tokenBucket.consumeWithBorrowNonBlocking(1.0, .0001, 1.0) == 0.0 ( 6241.23 == 0) @src/third_party/folly/test/token_bucket_test.cpp:47"}}
```

There may be a precision bug in the code-- I think it would also be acceptable to document the maximum precision of the rate if such a limit exists.

Start by running the CanConsumeFirstTokenLowRate test in src/third_party/folly/test/token_bucket_test.cpp and tracing consumeWithBorrowNonBlocking with the very low rate shown. Compare the returned value with the expected 0.0, then verify the behavior for nearby rates and determine whether the fix should preserve precision or document a rate limit.

Written by the indexing model from the issue text.