feat: Add public API for checking unhandled exceptions

[jamescrosswell]

Summary

Adds two new public extension methods to SentryEvent:

• IsFromUnhandledException(): Checks if the event is from any unhandled exception
• IsFromTerminalException(): Checks if the event is from a terminal (crash-causing) exception

Motivation

Fixes #2877

Users need to filter events in callbacks like BeforeSend based on whether exceptions are unhandled/terminal. This is particularly useful in MAUI apps where you might want to filter out common exceptions (like network timeouts) but always capture them if they're terminal.

Implementation

Following @bruno-garcia's feedback, this uses extension methods rather than making internal methods public. The methods check both Exception.Data flags and SentryExceptions mechanism properties.

Key differences:

• IsFromUnhandledException: Returns true for any unhandled exception (terminal or non-terminal)
• IsFromTerminalException: Returns true only for unhandled exceptions that are terminal (caused app crash)

Both are well-documented with XML docs and usage examples.

Exception.Data vs SentryException.Mechanism

These representations the same information at different pipeline stages.

• Exception.Data[Mechanism.HandledKey] is the input form. Unhandled-exception hooks set this in the raw System.Exception's Data dictionary.
• SentryException.Mechanism is what gets serialized and sent to Sentry.
• MainExceptionProcessor:171 converts the prior to the later

That's why the API checks both sources - depending on when someone calls it, the truth lives in a different place - and why the tests need to cover both variants.

They will never, in practice, be divergent/inconsistent so that's not a scenario we need to worry about.

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Changelog Entry

Add public API for checking unhandled exceptions (SentryEventExtensions.IsFromUnhandledException) and terminal exceptions (SentryEventExtensions.IsFromTerminalException)

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.17%. Comparing base (1d43862) to head (198a4c6).
⚠️ Report is 82 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Sentry/SentryEvent.cs	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5177      +/-   ##
==========================================
+ Coverage   74.08%   74.17%   +0.08%     
==========================================
  Files         506      509       +3     
  Lines       18247    18363     +116     
  Branches     3564     3590      +26     
==========================================
+ Hits        13519    13621     +102     
- Misses       3858     3869      +11     
- Partials      870      873       +3     

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