fix: ensure the region gets populated while site creation

[khushboo5723]

Problem

The Tokowaka dashboard recently started rendering a Region column for each
monitored site (project-elmo-ui-data commit cb98b3a, Apr 29). The column is
populated from Site.region returned by GET /sites, but ~84% of rows are
empty: of the 7 code paths in spacecat-api-service that call Site.create(),
only 2 (plg-onboarding.js and the Slack onboard command) currently invoke
locale detection. The remaining 5 paths create sites with region: "" and
language: "" — and have done so since the region attribute was added in
spacecat-shared-data-access PR #995 (Oct 2025).

This PR plugs those 5 holes so new sites going through any creation flow get
their locale detected at creation time.

Changes

New helper: src/support/locale.js

ensureSiteLocale(site, baseUrl, log) is the shared glue around
detectLocale from @adobe/spacecat-shared-utils:

1. Skips if site.getLanguage() and site.getRegion() are both already set
   (idempotent — won't overwrite caller-supplied values).
2. Calls detectLocale({ baseUrl }).
3. Sets site.setLanguage(...) / site.setRegion(...) only for fields that
   are currently empty AND for which detection returned a non-empty value.
4. Persists with site.save() if anything changed.
5. Swallows all errors (network, invalid URL, save failure) — locale
   enrichment is best-effort and must not block site creation.

Difference from the existing PLG/Slack-onboard pattern: the catch block
does NOT fall back to writing 'US'/'en'. When detection genuinely fails
(unreachable host, bot-blocked, timeout) the site is left with empty region
rather than fake-classified as US. Empty values are honest and can be filled
by a future backfill; fake values silently corrupt regional metrics.

5 call-sites wired

File	Location
src/controllers/sites.js	POST /sites
src/support/slack/commands/add-site.js	Slack add site
src/support/slack/actions/approve-site-candidate.js	Site candidate approval
src/controllers/llmo/llmo-onboarding.js	createOrFindSite (used by LLMO onboarding)
src/controllers/llmo/llmo.js	createOrUpdateStageEdgeConfig (stage-domain creation)

Each is a single await ensureSiteLocale(site, baseUrl, log); line right
after Site.create(...).

Tests

• New test/support/locale.test.js: 19 cases covering every branch of the
  helper at 100% coverage (verified via c8 --include='src/support/locale.js').
• 5 affected call-site test files updated to esmock the helper (matching the
  repo convention used in cdn-detection.test.js, plg-onboarding.test.js,
  etc.). Each updated test asserts the helper is invoked with the freshly
  created site and the correct base URL.

Out of scope (follow-ups)

1. detectLocale's internal 'US'/'en' fallback (in
   spacecat-shared-utils/src/locale-detect/locale-detect.js:61-62,
   pre-existing since PR fix(deps): update external fixes #1006). When fetch succeeds but no indicator
   (TLD/path/hreflang/htmlLang/headers/metaTag/content) fires, detectLocale
   returns {region:'US', language:'en'}. Our helper trusts that result, so
   plain .com sites with no localization markup still get classified as US.
   Proper fix is to add a { allowFallback: false } option upstream so callers
   can opt out — separate PR to spacecat-shared.
2. Backfill of historical empty-region sites. This PR fixes only sites
   created from now on. A one-off script that iterates Site.all(), calls
   ensureSiteLocale on each empty-region site, and saves will be a separate
   PR once we confirm the new write path works in stage.
3. Refactor plg-onboarding.js and support/utils.js to use the new
   helper instead of their inline duplicates of the same pattern. Deferred
   to keep this PR scoped; both already have the (problematic) 'US'/'en'
   catch fallback that we'd want to remove in lockstep.

Test plan

• Unit tests pass:
  npx mocha --spec='test/support/locale.test.js' --spec='test/controllers/sites.test.js' --spec='test/support/slack/commands/add-site.test.js' --spec='test/support/slack/actions/approve-site-candidate.test.js' --spec='test/controllers/llmo/llmo-onboarding.test.js' --spec='test/controllers/llmo/llmo.test.js'
  (the 14 pre-existing failures in the full repo suite — 9× detectedCdn
  Joi, 1× LlmoController before-all hook, 4× PLG timeouts — are
  unrelated to this change and reproduce on main)
• Stage smoke (curl POST /sites against stage):
  - https://www.bmw.de → region: "DE" (TLD signal)
  - https://www.toyota.co.jp → region: "JP"
  - https://www.lemonde.fr → region: "FR"
  - https://this-host-does-not-exist-xyz.invalid → region: "" (no fallback to US — this is the key behavior change)
• Stage Slack: @spacecat add site https://www.lemonde.fr, then
  @spacecat get site https://www.lemonde.fr → confirm region: "FR".
• Idempotency: POSTing the same site twice doesn't re-run detection or
  modify the saved record.
• User-supplied region honored: POST {baseURL: ..., region: "GB"} is
  not overwritten by detection.
• Day-after verification: tomorrow's llmo-usage-data-YYYY-MM-DD.json
  in project-elmo-ui-data/usage-tracking/ shows reduced empty-region
  rate for sites created in the last 24h.

[github-actions]

This PR will trigger a patch release when merged.

[codecov]

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