Support SPDX IDs with dots in license URL slugs

[mlaetitia]

Related to T51ENG-1496

Proposed changes

• Add dot-preservation for license post slugs (SPDX identifiers like Apache-2.0, GPL-3.0) across three layers: save-time (wp_insert_post_data, wp_unique_post_slug), query-time (sanitize_title filter with cached DB lookup), and REST API route regex.
• Add Gutenberg editor script that prevents cleanForSlug from stripping dots — handles both title-driven slug generation (wp.data.subscribe) and manual slug edits (capture-phase focusout listener).
• Fix negative cache lookup bug where false was indistinguishable from a cache miss.

Why are these changes being made?

WordPress sanitize_title strips dots from slugs by default. SPDX license identifiers use dots as version separators (e.g.,Apache-2.0), so license URLs were being mangled to /license/apache-2-0 instead of the canonical /license/apache-2.0. This caused SEO issues and broken links from external sources using the standard SPDX identifiers. The previous approach used the Redirection plugin which created conflicting canonical signals.

Testing instructions

• Create a new license post with title Apache-2.0 — slug should auto-generate as apache-2.0 (not apache-2-0)
• Edit an existing license post's slug to include dots (e.g., mit-1.0) — dots should persist after clicking away (brief visual
  flicker is expected)
• Visit /license/apache-2.0 on the front end — should resolve to the correct license post
• Visit /osi/v1/license/apache-2.0 REST endpoint — should return the license data
• Verify non-dotted license slugs (e.g., MIT) still work normally
• Verify other post types are unaffected by the sanitize_title filter