Validate BMLT root server on save (reject invalid/unreachable hostnames) [#282]

[dgershman]

Closes #282

Problem

The BMLT root server URL in Mayo settings was saved with only sanitize_text_field() — no check that it's a well-formed URL, let alone a real, reachable BMLT root server. A typo, a non-URL string, an http:// server, or a URL that isn't a BMLT root server was persisted silently and only surfaced later as broken/empty event lookups, with no indication that the root server was the cause.

Changes

• New RootServerValidator helper (includes/Rest/Helpers/RootServerValidator.php) — layered checks, cheapest first:
  1. Format — reject anything that isn't a valid absolute URL (esc_url_raw() + wp_http_validate_url()), normalized with no trailing slash.
  2. Scheme — require https:// (mirrors the existing client-side rule).
  3. Reachability — GET {url}/client_interface/json/?switcher=GetServerInfo via wp_remote_get() with a bounded 10s timeout; require HTTP 200 + JSON that looks like BMLT server info (a version field).
• Save path (SettingsController::update_settings) — validates only when the value changes, and returns early on failure before any update_option(), so the previously stored value is preserved. Saving unrelated settings never triggers a network call; clearing to empty is allowed.
• POST /validate-root-server (admin-only) + a "Test connection" button in the Settings UI to verify without saving.
• apiFetch now surfaces the REST error message so the clear validation error reaches the user.
• Changelog entry in readme.txt; regenerated languages/mayo-events-manager.pot for the new strings.

Acceptance criteria

• Saving a malformed / non-URL bmlt_root_server is rejected with a clear error; previous value preserved.
• Saving a well-formed URL that is not a reachable BMLT root server is rejected with a clear error.
• Saving a valid, reachable BMLT root server succeeds as today.
• Validation has a bounded (10s) timeout.
• Unit coverage for the validator (valid/reachable, malformed, non-https, unreachable, non-200, reachable-but-not-BMLT) plus save-path reject/preserve and the new endpoint.

Testing

• composer test — 526 tests pass (12 new).
• composer lint — clean.
• npm run build — compiles successfully.

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