Remove unreachable OOM handling

[azkrishpy]

Issue #, if available: N/A

Description of changes:

CRT allocators abort on failure rather than returning NULL. They route through AWS_PANIC_OOM, an unconditional exit(-1), so every OOM branch guarded by a NULL check was unreachable.

Removes those checks and the error paths behind them, and folds acquire-then-zero pairs into aws_mem_calloc. No assert replaces them: it would be dead in debug and compiled out in release, since the abort happens before control returns.

No behavior change. Existing suite passes unchanged.

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

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.85%. Comparing base (b32d9ee) to head (6bea455).

Files with missing lines	Patch %	Lines
source/proxy_connection.c	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            todos     #577      +/-   ##
==========================================
- Coverage   80.02%   79.85%   -0.18%     
==========================================
  Files          28       28              
  Lines       12105    12081      -24     
==========================================
- Hits         9687     9647      -40     
- Misses       2418     2434      +16     

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