fix(http2): reject websocket upgrades on non-200 responses

[trivikr]

This relates to...

Fixes: #5063

Rationale

client.upgrade() over HTTP/2 currently asserts when a websocket extended CONNECT request receives a non-200 response.

That assertion escapes the normal request error path and crashes the process instead of rejecting the upgrade promise.

Changes

• replace the upgrade-path assertion with a normal SocketError('bad upgrade') abort in the upgrade handler
• preserve existing successful upgrade behavior for H1 (101) and H2 (200)

Features

N/A

Bug Fixes

• reject HTTP/2 websocket upgrades with non-200 responses instead of throwing an uncaught assertion
• align H2 bad-upgrade handling with the existing upgrade error semantics used elsewhere in undici

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.10%. Comparing base (f6c5dda) to head (e93cff5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5072   +/-   ##
=======================================
  Coverage   93.10%   93.10%           
=======================================
  Files         110      110           
  Lines       35799    35805    +6     
=======================================
+ Hits        33329    33337    +8     
+ Misses       2470     2468    -2     

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[mcollina]

lgtm