RS-21803: checkImageTag style attribute test

[SurreyHughesDisplayr]

Summary

Adds unit test coverage for checkImageTag's handling of the style attribute and its surrounding link-cleaning paths (R/checkimagetag.R) — 7 test blocks in the existing test-checkimagetag.R.

• Covers a style attribute before and after src, exercising the two different quote/bracket cleaning paths rather than the same one twice.
• Covers double-quoted and unquoted src values, and src followed by trailing text.
• Covers the missing-src branch, pinning the echoed input in the warning so it is distinguishable from the tag-regex-miss branch that emits a byte-identical syntax error prefix.
• Asserts the success path wraps the original text, not the matched img tag.
• Each success block asserts the whole returned string in one expect_equal rather than several substring greps.

Also closes a pre-existing gap: the file's txt.dq case was single-quoted and identical to txt.sq, so double quotes were never actually tested. The style literal is now a single hoisted constant, replacing a dangling unused assignment.

Note on the src= regex

The last block pins current behaviour for a decoy input where an earlier attribute value contains src=. regexpr("src=(\S+)", ...) is unanchored, so alt='mysrc=x' is matched in preference to the real src, and a valid image is silently removed with a user-visible invalid link which has been removed: x warning. The test pins the warning text — deliberately, so that the assertion fails for the decoy and not merely because a network fetch failed, since the same warning is emitted on any failed GET.

This looks like a genuine low-severity defect rather than intended behaviour: nothing in the function or its callers suggests the loose match is deliberate, and a boundary requirement such as (?:^|\s)src= would be safe because the enclosing <img [^>]+> match guarantees whitespace before every attribute. Not fixed here; worth a ticket. Separately, the trailing-whitespace sub() in the same function is unreachable, since (\S+) cannot capture whitespace — inert rather than wrong.

Network dependency

The success-path blocks perform a live HTTP GET of a real wiki image URL — the pre-existing convention in this file, not introduced here. They fail offline. Documented in a comment at the top of the file so the failure mode is obvious to the next reader.

Jira: https://numbers.atlassian.net/browse/RS-21803

Stacked PR

Based on RS-21803-colwidthsandcustomcss (#71). Merge order: #66, #67, #68, #69, #70, #71, then this. This is the last of the seven RS-21803 plans; it is the only one touching checkimagetag.R rather than createcustomtable.R, so it shares no test file with the others.

Test plan

Rscript -e "devtools::load_all('.'); testthat::test_file('tests/testthat/test-checkimagetag.R')"

24 passing, 0 failures, 0 warnings (requires network access).

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