RedHerb integration probes cleanups (PR #781 follow-up)

[alistair3149]

Result of code review of #781 with @alistair3149.
Context: the NeoWiki codebase, the frontend-extensibility branch, and live verification of which findings were worth acting on.
_{Written by Claude Code, Opus 4.7 (1M context)}

Follows-up to #781

Targets redherb-sidebar-panel so this lands as a stacked review fix into #781 once approved.

Addressed

Eight commits, one per finding (read in branch order):

1. Rename ext.redherb-sidebar → ext.redherb-subject-finder — only loaded by Special:RedHerbSubjectFinder; old name was misleading.
2. Use fresh DerivativeContext + OutputPage in testRegistersResourceLoaderModules — was order-dependent on RequestContext::getMain().
3. SubjectFinderPanel.onSelected — add .catch() so a load failure logs and notifies instead of silently half-loading.
4. CreateChildDialog.loadSchema — .catch() now logs / notifies / closes instead of silently nulling state.
5. Move DIALOG_OPEN_KEY / DIALOG_STATE_KEY into per-module constants.js; init.js and .vue share via require() instead of duplicated string literals.
6. Add RedHerbFrontendModulesHookTest to pin the module-name contract.
7. Drop unused mediawiki.skin.variables.less imports from both dialog components.
8. Replace wfMessage(...) with $skin->msg(...) in RedHerbSidebarHook (context-aware lookup instead of RequestContext::getMain()); test stubs updated to mock Skin::msg() via a shared newSkinStub( Title $title ) helper that also dedupes the previous inline stubs.

Not addressed

• MAIN_SUBJECT_SELECTOR matches any rendered view, not specifically the main subject — needs either a public PHP API or a DOM marker, both intersecting with Establish public PHP API surface for extensions consuming NeoWiki services #789.
• Design-level findings split out to Establish public PHP API surface for extensions consuming NeoWiki services #789 (PHP public API surface) and Public TS helper for building schema-driven statement lists #790 (schema-driven statement-list helper).
• Two RedHerbSidebarHookTest cases (testOnlyAddsSubjectFinderLinkOnNonExistentPages and testDoesNotCheckMainSubjectForNonExistingTitles) cover the same $title->exists() === false path; not collapsed in this PR — judgment call left for the next pass.

Manual Browser Check

1. Visit Special:RedHerbSubjectFinder. Type a schema name, select a subject from the lookup, confirm the infobox renders.
2. On a content page with a main subject, click Create child Company in the sidebar. Fill the dialog, save. Confirm the success notification and the new subject on ?action=subjects.
3. Same page, click Edit main subject. Change the label, save. Confirm the existing infobox re-renders without reload.
4. Confirm the browser network tab on the special page loads ext.redherb-subject-finder (renamed), not ext.redherb-sidebar.
5. Optional error path: take the API offline and load the special page; pick a subject — confirm an error notification fires instead of a silent half-load.

Verification

Local run on this branch (NeoWiki dev container, MW REL1_43, PHP 8.3):

• make phpunit — OK, 858 tests, 1636 assertions, 7 skipped (28s)
• vendor/bin/phpcs — clean, 328 files (5s)
• vendor/bin/phpstan — [OK] No errors, 150 files

Browser pass (Chrome DevTools, against the running dev wiki on this branch):

• ✅ Step 1 — Special:RedHerbSubjectFinder with Schema = Company, picked ACME Inc., the Infobox renders all properties (Founded at, Websites, Products, Status, World domination progress).
• ✅ Step 2 — Create child Company opens the SubjectEditor-backed dialog; Status = Active, label Test Subsidiary, Create returns 201 from POST /rest.php/neowiki/v0/page/24/childSubjects; dialog auto-closes; ?action=subjects shows Test Subsidiary Company • 1 statement.
• ✅ Step 3 — Edit main subject opens pre-populated with all ACME Inc. statements (label, founded, websites, products, status, progress). Changed label to ACME Incorporated; on save the page-side Infobox <h2> reactively updates from ACME Inc. → ACME Incorporated with no reload, and Updated main subject. mw-notify fires. This is the cross-app shared-Pinia path Add frontend-extensibility probes to RedHerb #781 is meant to prove out.
• ✅ Step 4 — network shows load.php?...modules=ext.redherb-create-child,redherb-edit-main-subject,redherb-subject-finder...; no stale ext.redherb-sidebar reference.
• Step 5 (offline error path) skipped — invasive on the dev environment; the unit-tested .catch() handlers in commits 3 and 4 cover the same code path.