Search: fix uncaught URIError from malformed query params crashing Instant Search

[interlineae]

Fixes #50709

Proposed changes

• Guard decodeURIComponent() in Instant Search's vendored query-string decoder (query-string-decode.js) against malformed percent-encoding (e.g. a stray %, an unresolved email merge tag like %DONOR%, or invalid escapes like %zz), which previously threw an uncaught URIError on every page load and crashed Instant Search's initialization, leaving the page blank.
• Malformed sequences are sanitized (a bare % not starting a valid %XX escape is escaped to %25) and then decoded, so the value is recovered as literal plain text (e.g. %DONOR% decodes to the literal string %DONOR%) instead of throwing. This lets Instant Search keep treating the value as an active search term (e.g. showing its own "No results found" state) rather than silently dropping it or falling back to the site's classic search page.
• In the rare residual case of a value that's well-formed %XX escapes but decodes to an invalid byte sequence (e.g. a truncated UTF-8 sequence), that key is dropped rather than kept as a raw value — keeping a still-encoded raw string would get double-encoded (and corrupted) the next time the query object is round-tripped through qss's encode().

Related product discussion/links

• [Jetpack Search] Malformed query param value throws uncaught URIError and blanks the page #50709

Does this pull request change what data or activity we track or use?

No.

Testing instructions

• On a site with Jetpack Search active and the Instant Search (overlay) experience enabled, visit a URL with a malformed query param typed directly in the address bar (not submitted via a form, which would properly encode it), e.g.:
  • https://yoursite.com/?s=%MA
  • https://yoursite.com/?s=%zz
  • https://yoursite.com/?test=%MALFORMED%
• Before this fix: the page renders blank, and the console shows an uncaught URIError: URI malformed.
• After this fix: the page renders normally. For a malformed s (search term) param, the Instant Search overlay opens showing "No results found" for the literal text (e.g. %MA). For a malformed bystander param unrelated to search, the page loads normally and that param is otherwise unaffected.
• Run the JS test suite: pnpm test-scripts (from projects/packages/search/) — includes new regression tests in src/instant-search/external/test/query-string-decode.test.js.

[github-actions]

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[github-actions]

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/search/src/instant-search/external/query-string-decode.js	18/21 (85.71%)	54.14%	-10 💚

Full summary · PHP report · JS report

[robfelty]

Looks good with one change. I don't think we should try to do the bare % replacement, because it results in confusing behavior, where it works sometimes and not others, in a way that is likely to make debugging more difficult in the future. I think catching the error is sufficient.