fix: revise format-handling logic in noop.ts and service.ts

[eya46]

After upgrading to the latest Astro (>=5.15.9), all SVG images stopped displaying.

Issues found:

• With passthroughImageService enabled, SVG images return Content-Type: image/null and fail to render (JPG works).

After downgrading Astro, the regression starts at 5.15.9.
Related issues:

• passthroughImageService() does not prevent WebP conversion for Image component in Astro 5.15.2 #14721: images incorrectly converted to WebP
• fix: passthroughImageService generate webp #14776: introduced validateOptions in noop.ts to address it

delete newOptions.format; -> "Content-Type": mime.lookup(format) ?? `image/${format} -> Browser Content-Type: image/null -> image boom (svg)

Changes

I believe the issue in #14721 originates from the options.format = DEFAULT_OUTPUT_FORMAT assignment in service.ts.
To address this, I added a flag to prevent setting a default output format when the noop service is used.

Although my JPG images still return image/null (but browsers can display them), SVG images now work correctly.

before:

after:

Testing

I wasn’t sure how to write it.

Docs

There shouldn't be much to change. This aligns better with the note in #configure-no-op-passthrough-service
stating that Note that Astro does not perform any image transformation and processing in these environments.

[changeset-bot]

🦋 Changeset detected

Latest commit: 8a10f23

The changes in this PR will be included in the next version bump.

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[codspeed-hq]

CodSpeed Performance Report

Merging #14861 will not alter performance

_{Comparing eya46:fix-noop-bug (8a10f23) with main (78ed8b5)¹}

Summary

✅ 6 untouched

Footnotes

1. No successful run was found on main (7d67434) during the generation of this report, so 78ed8b5 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩