test: update duplicate many_digits test to use f64 instead of f32

[lemire]

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 #86761 (comment) 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.

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer