[yyuu]

Summary

• Start WEBrick servers only when a test first needs them.
• Join listener threads during teardown and continue cleanup if client teardown raises.
• Add focused lifecycle tests and remove redundant server setup from unrelated tests.

The original intermittent failure was not reproduced. This PR isolates the fixture lifecycle changes for review.

Testing

• Local suite: 262 tests, 886 assertions, no failures or errors (8 omissions), on Ruby 3.4.7.
• GitHub Actions: all 12 Ruby/JRuby matrix jobs passed on the first attempt.

[yyuu]

This only touches test code, and at least it is not contributing the stability worse so far. I'm planning to proceed for merging as it is.