GCPSkillRegistry rejects every Google-published skill: dotted ids (cloud.google.com-*) fail name validation in get_skill and search_skills

Follow-up to #6838 / #6839: that fix stopped search_skills from crashing on these ids, but the ids themselves are still rejected everywhere, so Google-published skills remain unusable from ADK.

🔴 Required Information

Describe the Bug:
Skills published by Google into Agent Registry have resource ids of the form cloud.google.com-<display-name> (also discoveryengine.googleapis.com-<name>). ADK cannot use any of them:

• GCPSkillRegistry.get_skill(name="cloud.google.com-google-cloud-networking-observability") raises ValueError: Invalid skill name ... because the name check added in 11101acc only accepts kebab/snake case (no dots).
• GCPSkillRegistry.search_skills() feeds each id into Frontmatter(name=...), whose validator also rejects dots, so since 3c977bc2 (#6838) every Google-published hit is dropped with a "Skipping search result" warning. tests/unittests/integrations/skill_registry/test_gcp_skill_registry.py even uses cloud.google.com-agent-platform-eval-flywheel as the example of a "bad" name.

In a real project catalog (location global) this rejects 113 of 117 skills; the only 4 that pass are ones we created ourselves with plain kebab ids. With the validation bypassed locally, downloading the Google skill works (SKILL.md with 7184 chars, 8 resources; the frontmatter name inside the archive equals the displayName and is valid kebab-case), so the archives themselves are fine. It is only the registry id that fails the SKILL.md naming rule.

Steps to Reproduce:

1. google-adk main (ac0133a4) or 2.9.1, project with Agent Registry enabled (Google-published skills are visible in the Console).
2. registry = GCPSkillRegistry(project_id=..., location="global")
3. await registry.search_skills(query="networking") -> only self-created skills are returned; one warning per Google skill.
4. await registry.get_skill(name="cloud.google.com-google-cloud-networking-observability") -> ValueError.

Expected Behavior:
Google-published skills are searchable and loadable. The security intent of 11101acc (single path segment, no traversal) can be kept by validating the id as a safe path segment (e.g. ^[A-Za-z0-9._-]+$, and not ./..) instead of applying the SKILL.md frontmatter naming rule to a registry resource id. search_skills should not validate registry ids with the frontmatter rule either; that rule belongs to the SKILL.md inside the archive, which does pass.

Observed Behavior:
ValueError from get_skill, and silent exclusion from search_skills.

Environment Details:

• ADK Library Version: main @ ac0133a4, also 2.9.1
• Reproduced locally (macOS, Python 3.14) against a real Agent Registry catalog

Model Information:

• Are you using LiteLLM: No
• Which model is being used: N/A (registry client only)

Additional Context

• #6838 reported this exact catalog entry (cloud.google.com-agent-platform-eval-flywheel) but asked only for the crash to be fixed; #6839 implemented "skip and log", and its description states this matches get_skill() already rejecting those names. So the current behaviour is deliberate at the symptom level, but it means every Google-published skill (113/117 in our catalog) is unreachable from ADK, which I don't think was the intent. This issue is about accepting those ids, not about the crash.
• Related: #6908 / #6824 (redirect on media download; needed for the download to succeed at all), #7130 / #7135 (pinning registry skills). Note that registry id and frontmatter name differ for 117/117 skills in the catalog, which is why #7135 needs to key the pinned skill by both names.
• Happy to send a PR.