fix(douyin): search hashtags through the live endpoint and report empty responses

[Benjamin-eecs]

Description

douyin hashtag search failed for everyone: creator.douyin.com/aweme/v1/challenge/search/ answers HTTP 200 with an empty body, and the command reported Douyin API error -2 ... JSON parse failed, which pointed #2205's reporter at the JSON rather than the endpoint.

The creator studio composer reads hashtag suggestions from /aweme/v1/search/challengesug/ instead; it carries the same cha_name / cid / view_count fields in sug_list and returns a fixed-size list, so the rows are capped by --limit here.

The empty-body branch in browserFetch also comes back (#1408 added it, #1587 lost it to a generic parse-failure wrapper): an empty or whitespace-only 2xx body raises Empty response from Douyin API with a hint naming the usual causes, which also revives the publish flow's fast_detect retry matching on that text; a non-2xx empty body keeps its status, so 401 and 403 stay AuthRequiredError. The old tests mocked page.evaluate wholesale so the generated script never ran; the new ones execute it against a stubbed fetch.

Related issue: Closes #2205

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🌐 New site adapter
• 📝 Documentation
• ♻️ Refactor
• 🔧 CI / build / tooling

Checklist

• I ran the checks relevant to this PR
• I updated tests or docs if needed
• I included output or screenshots when useful

Documentation (if adding/modifying an adapter)

• Added doc page under docs/adapters/ (if new adapter)
• Updated docs/adapters/index.md table (if new adapter)
• Updated sidebar in docs/.vitepress/config.mts (if new adapter)
• Updated README.md / README.zh-CN.md when command discoverability changed
• Used positional args for the command's primary subject unless a named flag is clearly better
• Normalized expected adapter failures to CliError subclasses instead of raw Error

Screenshots / Output

Before, on a logged-in creator session:

$ opencli douyin hashtag search --keyword 美食
error:
  code: COMMAND_EXEC
  message: 'Douyin API error -2 at GET .../challenge/search/...: JSON parse failed: Unexpected end of JSON input'

After, same session:

$ opencli douyin hashtag search --keyword 美食 --limit 5
- name: 美食
  id: '1554036363744257'
  view_count: 1052034193110
- name: 美食教程
  id: '1581688162541582'
  view_count: 422585850860

An endpoint that does answer empty now says so, with the URL and a hint instead of a parse error, and --limit 0 is rejected up front. clis/douyin suite 195 / 195 with twelve new tests, one running the generated script under the bridge envelope; dropping the composer parameters, mapping before the limit slice, or discarding the HTTP status on an empty body each fails a test. Typecheck, both lint gates and doc coverage pass; cli-manifest.json unchanged.

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