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.

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[Mark-Simulacrum]

Any reason to replace the test rather than adding a new one with the f64 case? That seems better to me, though I'm not familiar with this particular area of code.

[lemire]

Any reason to replace the test rather than adding a new one with the f64 case? That seems better to me, though I'm not familiar with this particular area of code.

There were two f32 tests, but really just one duplicated.

This PR replaces one of the f32 tests by the equivalent f64 test, thus improving coverage.

It seems that this PR was the original goal.