fix(cli): report GitHub API rate limits in difyctl install scripts

[lin-snow]

Summary

The difyctl install scripts resolve releases through the GitHub REST API with curl -fsSL. -f discards the response body and the X-RateLimit-* headers, so when the API rate limit is hit (60 req/hour per unauthenticated IP — common behind shared NATs, corporate egress, or in CI) the failure was misreported as Dify release <x> not found or failed to query latest Dify release, sending users to debug the wrong thing.

This PR makes the scripts detect and explain rate limiting:

• fetch_json now reads the HTTP status and rate-limit headers and classifies the failure (rate limit vs. 404 vs. network) instead of collapsing everything into one opaque exit code.
• On a rate limit it prints an actionable hint: the cause, the reset ETA, and how to proceed.
• GITHUB_TOKEN / GH_TOKEN, when set, is sent as a bearer token to raise the limit from 60 to 5000 req/hour.
• Mirrored in install.ps1; unit tests added for both.

The success path is unchanged, and non-rate-limit errors keep their original messages.

Screenshots

N/A — CLI install-script change. Example new output when rate limited:

install-cli: GitHub API rate limit exceeded (unauthenticated requests are capped at 60/hour per IP).
install-cli: The limit resets in ~30 min.
install-cli: To proceed now, authenticate to raise the limit to 5000/hour:
install-cli:   curl -fsSL <install-url> | GITHUB_TOKEN=<token> sh

Checklist

• This change requires a documentation update, included: Dify Document — the public install page's GITHUB_TOKEN note is a follow-up; cli/README.md is updated here.
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly. (cli/README.md)
• I ran the cli/ gates instead of the backend/frontend ones (not applicable here): pnpm lint, pnpm type-check, and vitest on the changed suites — all green.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.34%. Comparing base (5622e8f) to head (b2cb47f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #38375      +/-   ##
==========================================
+ Coverage   85.29%   85.34%   +0.05%     
==========================================
  Files        4992     5147     +155     
  Lines      264162   268493    +4331     
  Branches    50126    51184    +1058     
==========================================
+ Hits       225309   229140    +3831     
- Misses      34434    34934     +500     
  Partials     4419     4419              

Flag	Coverage Δ
cli	88.45% <ø> (?)

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[GareArc]

lgtm