feat: use GitHub Git Refs API for efficient tag validation

[suztomo]

Replaced the 1100-tag scan with a direct lookup using the Git Refs API.
This resolves flakiness for large repositories like Genkit and improves
efficiency by reducing the number of API calls.

b/440379162

GitHub API is lenient about the URL encoding

I thought about whether GitHub API needs URL encoding for the tags that have "@" character. It turned out it's not needed.

GitHub API not requiring URL encoding

suztomo@suztomo:~/b468377909/wombat-dressing-room$ gh api --verbose "/repos/firebase/genkit/git/ref/tags/genkitx-pinecone@1.27.0-rc.2"
* Request at 2025-12-26 18:16:01.080415371 +0000 UTC m=+0.085464534
* Request to https://api.github.com/repos/firebase/genkit/git/ref/tags/genkitx-pinecone@1.27.0-rc.2
> GET /repos/firebase/genkit/git/ref/tags/genkitx-pinecone@1.27.0-rc.2 HTTP/1.1

...

{
  "ref": "refs/tags/genkitx-pinecone@1.27.0-rc.2",
  "node_id": "REF_kwDOL1BamNoAJnJlZnMvdGFncy9nZW5raXR4LXBpbmVjb25lQDEuMjcuMC1yYy4y",
  "url": "https://api.github.com/repos/firebase/genkit/git/refs/tags/genkitx-pinecone@1.27.0-rc.2",
  "object": {
    "sha": "22a48d246bf301448ab775094af9df35dcb9f14a",
    "type": "commit",
    "url": "https://api.github.com/repos/firebase/genkit/git/commits/22a48d246bf301448ab775094af9df35dcb9f14a"
  }
}

* Request took 152.090625ms