fix: make attachment payloads replayable

[XAN9xXx]

Summary

Fixes #5499.

When Spotlight is enabled, the same envelope can be serialized by both the
primary transport and the Spotlight transport. Attachment payloads previously
shared a single stream, so one serialization could consume the content before
the other transport read it.

This change makes attachment payloads replayable:

• Built-in byte attachments obtain an independent stream for each
  serialization.
• Built-in reusable file attachments open an independent file stream for each
  serialization.
• Stream-backed, custom or derived attachment content, and delete-on-close
  files are buffered in memory on first serialization and reuse that buffer
  for subsequent serializations.

Implementation notes

The buffering path obtains the source stream while creating the envelope item
for length validation, but does not read its content until the first
serialization. The resulting buffer is shared by repeated and concurrent
serializations.

Exact type checks are used for the built-in byte and file attachment types
because derived or custom implementations may return streams with different
lifetime or replayability behavior.

Envelope and transport ownership semantics are unchanged.

Testing

Regression coverage includes:

• Repeated byte, file, and stream attachment serialization
• Concurrent serialization
• Synchronous and asynchronous serialization
• Non-seekable and asynchronous-only streams
• Custom and derived attachment content
• Delete-on-close file attachments
• Lazy buffering and cancellation behavior

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[codecov]

Codecov Report

❌ Patch coverage is 77.04918% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.74%. Comparing base (5cd6ad6) to head (a302317).

Files with missing lines	Patch %	Lines
src/Sentry/Protocol/Envelopes/Envelope.cs	56.25%	3 Missing and 4 partials ⚠️
...entry/Protocol/Envelopes/AttachmentSerializable.cs	57.14%	3 Missing ⚠️
...y/Protocol/Envelopes/BufferedStreamSerializable.cs	84.21%	3 Missing ⚠️
src/Sentry/Protocol/Envelopes/EnvelopeItem.cs	93.75%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5511      +/-   ##
==========================================
- Coverage   74.74%   74.74%   -0.01%     
==========================================
  Files         513      515       +2     
  Lines       18829    18887      +58     
  Branches     3682     3690       +8     
==========================================
+ Hits        14074    14117      +43     
- Misses       3875     3885      +10     
- Partials      880      885       +5     

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