chore: check class @example blocks in check_docs

[tomas-zijdemans]

check_docs.ts never validated @example blocks on classes. assertClassDocs only called assertHasExampleTag when the class had no @example tag at all, so any class that did have one skipped the title check and the assertion-import check. Functions, methods and properties always got both. The guard came in with #6491.

This PR calls assertHasExampleTag unconditionally. That surfaced four class docs that fail, which I fixed here too:

• async/unstable_semaphore.ts: the Semaphore example has no assertion. I marked it no-assert, like every member example in the file.
• cli/unstable_progress_bar.ts: the "Custom Formatting" example was missing its closing fence, so the code block never ended. This one was a real rendering bug.
• internal/assertion_state.ts: the AssertionState example only constructs the object. I marked it no-assert to match its members.
• streams/unstable_capped_delimiter_stream.ts: the @example had no title, so it rendered as "Example 1". It's now "Usage".

Testing: deno task lint:docs passes. It reported 0 errors before the checker change and exactly these 4 with it. deno test --doc on the four touched modules passes (19 tests).

I used Claude Code to help investigate and write this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.04%. Comparing base (f834d02) to head (64af2e8).

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #7339    +/-   ##
========================================
  Coverage   95.04%   95.04%            
========================================
  Files         619      618     -1     
  Lines       52012    51757   -255     
  Branches     9450     9401    -49     
========================================
- Hits        49434    49192   -242     
+ Misses       2031     2022     -9     
+ Partials      547      543     -4     

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