feat!: add support for crash-time attachments via hint

[jpnurmi]

Merge attachments and pass as hint to on_crash, then use the resulting hint attachments in the envelope.

Caution

BREAKING CHANGE: on_crash callbacks now have an additional sentry_hint_t *hint parameter.

Close: #2098

[cursor]

Cursor Bugbot has reviewed your changes and found 3 potential issues.

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[codecov]

Codecov Report

❌ Patch coverage is 60.41667% with 76 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.35%. Comparing base (ce10a77) to head (d8f1a2a).

Additional details and impacted files

@@                  Coverage Diff                   @@
##           jpnurmi/feat/hints    #2112      +/-   ##
======================================================
+ Coverage               75.20%   75.35%   +0.14%     
======================================================
  Files                     103      104       +1     
  Lines                   27936    28069     +133     
  Branches                 5102     5126      +24     
======================================================
+ Hits                    21008    21150     +142     
+ Misses                   5600     5587      -13     
- Partials                 1328     1332       +4     

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[sentry]

Bug: The early-return guard sentry__atomic_fetch(&state->crashed) in native_backend_flush_scope prevents scope-level attachments added from an on_crash_func callback from being written, as the crashed state is set before the callback runs.
_{Severity: HIGH}

Suggested Fix

Revert the order of operations in native_backend_flush_scope. The attachment manifest should be written before checking the state->crashed flag. This ensures that attachments added from within an on_crash_func are correctly persisted, restoring the behavior that was explicitly supported in the previous version.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: src/backends/sentry_backend_native.c#L1240-L1243

Potential issue: In `native_backend_flush_scope`, the new early-return guard checks
`sentry__atomic_fetch(&state->crashed)` before writing the attachment manifest. The
`crashed` state is set before the `on_crash_func` is invoked. Consequently, if
`sentry_add_attachment()` is called from within an `on_crash_func` callback, it triggers
`native_backend_flush_scope`, which now returns early. This causes the scope-level
attachment to be silently dropped. The previous implementation intentionally wrote the
manifest before this check to support this exact use case, so this change is a
regression.

_{Did we get this right? 👍 / 👎 to inform future reviews.}