test: update duplicate many_digits test to use f64 instead of f32
Replace the f32 test case with an f64 equivalent to improve coverage for parsing large digit counts in double-precision floating-point conversion. Specifically, this PR updates the `many_digits` test in `library/coretests/tests/num/dec2flt/parse.rs` to test f64 (double-precision) parsing instead of f32 (single-precision).
The test verifies that decimal strings with an excessive number of digits (beyond `Decimal::MAX_DIGITS`) are parsed correctly, ensuring proper truncation of insignificant digits. Previously, the same test was repeated twice (see comment https://github.com/rust-lang/rust/pull/86761#issuecomment-3623334228 by `@Viatorus).`
## Changes
- Replaced the duplicated f32 test case with an equivalent f64 test case.
- Updated the expected bit pattern and input string to a very long decimal with many trailing zeros, testing the limits of f64 precision.
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