Replace strict-aliasing UB in BSON double serialization

[metsw24-max]

Replace reinterpret_cast-based type punning in BufferHelper::writeDouble with a standards-compliant std::memcpy bit copy.

The previous implementation reinterpreted a double as an int64_t through a pointer cast, which violates C++ strict-aliasing rules and may result in undefined behavior under compiler optimization.

This change preserves the exact serialized bit pattern while removing UB from the BSON double serialization path. A static_assert was also added to verify size compatibility between double and int64_t.

Risk Level:

Low

Testing:

• Verified bit-pattern equivalence for representative edge cases:
  zero, signed zero, ±1, π, ±∞, NaN, denormals, and ±DBL_MAX
• Verified round-trip correctness through writeDouble/removeDouble

Docs Changes:

None

[repokitteh-read-only]

Hi @metsw24-max, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #44975 was opened by metsw24-max.

see: more, trace.

[Mythra]

The actual code itself looks fine to me, though the CI system retriggers are interesting, have you been able to get tests passing locally @metsw24-max (i'll be running them locally myself here)? I'm just a bit surprised this PR has had force pushes/ci retriggers so often.

[kyessenov]

See the warning in CI:

ERROR: ./source/extensions/filters/network/mongo_proxy/bson_impl.cc:138: Don't call memcpy() directly; use safeMemcpy, safeMemcpyUnsafeSrc, safeMemcpyUnsafeDst or MemBlockBuilder instead.

[tyxia]

@metsw24-max Please fix the presubmit failure

/wait