io.fits.Card: shortest-roundtrip float formatting [skip ci] (swev-id: astropy__astropy-14508)

[rowan-stein]

Summary

• Fix float formatting in astropy.io.fits Card to avoid unnecessary expansion (e.g., 0.009125 -> 0.009124999999999999) that causes comment truncation.
• Use Python’s shortest-roundtrip str() for finite floats, normalize exponent to 'E'. If str(value) exceeds 20 chars, attempt a compact G-format fallback that still round-trips; otherwise keep str(value).

Issue

• io.fits.Card: avoid unnecessary float expansion; use shortest-roundtrip str for floats #114

Observed failure (before fix)

• Creating a Card with value 0.009125 expanded to 0.009124999999999999 and emitted VerifyWarning: comment truncated.

Reproduction

from astropy.io import fits
from astropy.io.fits.verify import VerifyWarning
import warnings
with warnings.catch_warnings(record=True) as caught:
    warnings.simplefilter("always", VerifyWarning)
    card = fits.Card(
        "HIERARCH ESO IFM CL RADIUS",
        0.009125,
        "[m] radius arround actuator to avoid",
    )
print([str(w.message) for w in caught])
print(str(card))

Output (pre-fix):

warnings: ["Card is too long, comment will be truncated."]
HIERARCH ESO IFM CL RADIUS = 0.009124999999999999 / [m] radius arround actuator 

Validation (after fix)

• Warnings: []
• String: HIERARCH ESO IFM CL RADIUS = 0.009125 / [m] radius arround actuator to avoid

Tests added

• Regression for 0.009125 with full comment.
• Boundary values: (1 - 2**-53) * 2**exp for exp in [-60, 0, 60].
• Special tokens preserved for NaN/Inf/-Inf.
• Long representation keeps shortest-roundtrip str for sys.float_info.max.

Notes

• Title includes required token. Base branch: astropy__astropy-14508. CI is skipped via commit message.

[noa-lucent]

Changes look solid. Thanks.