Fix _Once.ensure() to propagate handshake failure to concurrent waiters

[bysiber]

Problem

_Once.ensure() in SSLStream can leave concurrent waiters hanging forever when the handshake fails.

When two tasks share an SSLStream (one sending, one receiving), both call ensure() to lazily perform the TLS handshake. The first task sets started = True and begins the handshake. The second task sees started=True, finds _done not yet set, and enters _done.wait().

If the handshake fails (certificate error, connection reset, etc.), the exception propagates to the first task — but _done.set() is never called. The second task is stuck forever in _done.wait(): the Event will never be signalled, and started is permanently True, so re-entry won't help either.

Reproduction scenario

1. Task A calls send_all() → enters ensure(), starts handshake
2. Task B calls receive_some() → enters ensure(), waits on _done
3. Handshake fails (remote peer rejects cert)
4. Task A gets BrokenResourceError — correct
5. Task B hangs indefinitely

Fix

Store the exception on failure and still signal _done, so that concurrent waiters (and any future callers) wake up and receive a BrokenResourceError chained from the original handshake exception.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.97942%. Comparing base (2fd138e) to head (aa3256a).

Files with missing lines	Patch %	Lines
src/trio/_ssl.py	66.66667%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@                 Coverage Diff                  @@
##                 main       #3397         +/-   ##
====================================================
- Coverage   100.00000%   99.97942%   -0.02058%     
====================================================
  Files             128         128                 
  Lines           19424       19434         +10     
  Branches         1318        1320          +2     
====================================================
+ Hits            19424       19430          +6     
- Misses              0           2          +2     
- Partials            0           2          +2     

Files with missing lines	Coverage Δ
src/trio/_ssl.py	98.14815% <66.66667%> (-1.85185%)	⬇️

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[A5rocks]

Could you create an issue before making a PR fixing a bug like this? I'd like to confirm that this is a real problem first!

[A5rocks]

This is a bad idea, because a) the stack frames for exc will be mutated, so you will have weird stack traces for raise ... from self._failure and b) this will lead to a refcycle I believe.