Add integration coverage for foster-parented table siblings

[kitsuyui]

Summary

• add an integration helper that inspects element child sequences
• cover malformed table content that triggers foster parenting before a table sibling
• assert that existing comment, foster-parented text/element nodes, and trailing table siblings keep their expected order

Why

append_before_sibling rebuilds sibling lists around table foster-parenting boundaries. This adds a public-API regression test for the reachable path that is most likely to lose or reorder siblings during refactors.

Testing

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo check
• cargo test --all-targets --all-features

[github-actions]

Code Metrics Report

	main (b206356)	#123 (ebcb43d)	+/-
Coverage	69.0%	81.3%	+12.2%
Code to Test Ratio	1:0.1	1:0.2	+0.0
Test Execution Time	6s	5s	-1s

Details

  |                     | main (b206356) | #123 (ebcb43d) |  +/-   |
  |---------------------|----------------|----------------|--------|
+ | Coverage            |          69.0% |          81.3% | +12.2% |
  |   Files             |              4 |              4 |      0 |
  |   Lines             |            605 |            605 |      0 |
+ |   Covered           |            418 |            492 |    +74 |
+ | Code to Test Ratio  |          1:0.1 |          1:0.2 |   +0.0 |
  |   Code              |            652 |            652 |      0 |
+ |   Test              |             71 |            134 |    +63 |
+ | Test Execution Time |             6s |             5s |    -1s |

Code coverage of files in pull request scope (68.9% → 81.4%)

Files	Coverage	+/-	Status
src/handle.rs	52.3%	+34.9%	affected
src/lib.rs	84.8%	+8.6%	affected
src/util.rs	85.2%	+26.4%	affected

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Reported by octocov