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

Changelog Entry

fix: Attachments not being sent properly when Spotlight is enabled

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[codecov]

Codecov Report

❌ Patch coverage is 92.06349% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.76%. Comparing base (5cd6ad6) to head (35609ec).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Sentry/Protocol/Envelopes/Envelope.cs	75.00%	2 Missing and 2 partials ⚠️
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%   74.76%   +0.01%     
==========================================
  Files         513      515       +2     
  Lines       18829    18884      +55     
  Branches     3682     3688       +6     
==========================================
+ Hits        14074    14118      +44     
- Misses       3875     3883       +8     
- Partials      880      883       +3     

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[jamescrosswell]

Thanks @XAN9xXx - I added a couple of commits to address the disposal/ownership issues but otherwise looks great. Thanks again for the contribution!

[XAN9xXx]

Appreciate you catching the oversight, and thanks for the review! Glad to contribute.