Fix stale key_method pointer after private key switch in CERT

[justsmth]

Description of changes:

key_method and per-slot privatekey on CERT are mutually exclusive — key_method
wins when non-NULL. But several paths didn't clear the other when setting one, so stale
values could silently take effect after switching between a real key and a callback.

Adds SetKeyMethod and SetSlotPrivateKey on CERT that enforce the invariant, and
routes all mutation paths through them.

Call-outs:

• dc_key_method / dc_privatekey have the same issue — left for a follow-up.
• SSL_set_private_key_method / SSL_CTX_set_private_key_method stay void for API
  compat; failures go to the error stack.

Testing:

Added SSLTest.KeyMethodPrivateKeyMutualExclusivity — 6 sub-cases covering invariant
checks across all mutation paths and functional handshake checks after key switches in
both directions.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.60759% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.18%. Comparing base (f3c6fff) to head (e9589e6).

Files with missing lines	Patch %	Lines
ssl/ssl_cert.cc	73.68%	5 Missing ⚠️
ssl/ssl_privkey.cc	50.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3085      +/-   ##
==========================================
- Coverage   78.36%   78.18%   -0.19%     
==========================================
  Files         689      689              
  Lines      121743   121814      +71     
  Branches    16997    16995       -2     
==========================================
- Hits        95402    95238     -164     
- Misses      25452    25692     +240     
+ Partials      889      884       -5     

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