Preserve block-less at-rules when keep_at_rules is enabled

[jmortlock]

Problem

With keep_at_rules: true, an at-rule terminated by a semicolon instead of a block — @import being the common case — loses its prelude and terminator. AtRuleFilteringParser writes @ + the rule name in parse_prelude and appends the prelude/body in parse_block, but never implements rule_without_block (whose default rejects the rule), so the already-written bare @import fuses with the following rule:

/* input */
@import url("print.css");
@media (max-width: 600px) { h1 { font-size: 18px; } }

/* output */
@import@media (max-width: 600px) { h1 { font-size: 18px; } }

Browsers treat the fused text as one invalid rule and discard it — in Chromium the resulting <style> sheet parses to cssRules.length === 0 — so a single @import silently disables every at-rule the option was meant to keep. We hit this inlining MJML email output, where the responsive @media block stopped applying on mobile clients.

Fix

Implement rule_without_block to emit the prelude and a terminating semicolon, mirroring how parse_block emits blocked at-rules:

@import url("print.css"); @media (max-width: 600px) { h1 { font-size: 18px; } }

Includes a regression test (fails on master with the fused output above) and a changelog entry. Full suite passes locally apart from the four tests that need the 127.0.0.1:1234 stylesheet server.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.05%. Comparing base (5142700) to head (62a1e25).

Files with missing lines	Patch %	Lines
css-inline/src/html/selectors/attr_value.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #768      +/-   ##
==========================================
+ Coverage   92.02%   92.05%   +0.02%     
==========================================
  Files          17       17              
  Lines        2584     2593       +9     
==========================================
+ Hits         2378     2387       +9     
  Misses        206      206              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[codspeed-hq]

Merging this PR will not alter performance

✅ 12 untouched benchmarks

---

_{Comparing jmortlock:fix-keep-at-rules-without-block (62a1e25) with master (5142700)}

[Stranger6667]

Awesome! Thank you :)

Could you please add the same changelog entry to all the bindings? as it is a user-visible change for non-rust users

[jmortlock]

Done — added the same entry under [Unreleased] in all six binding changelogs (Python, Ruby, Java, C, PHP, JavaScript), using each binding's own option spelling (keep_at_rules / keepAtRules).