(Improvement): Capture richer rspec exception details

[TylerJang27]

Summary

Improving what rspec_trunk_flaky_tests captures for failed examples so the data we report matches what users see in their RSpec console output.

• failure_message now delegates to RSpec::Core::Formatters::ExceptionPresenter#fully_formatted (with a no-op colorizer). That gives us the full Failure/Error: <source line> header, the exception class and message, and any Caused by: chain — instead of just <class>: <message>. Previously the source line and root-cause chain were silently dropped.
• backtrace uses RSpec's formatted_backtrace (which applies the configured backtrace cleaner) and walks exception.cause so each cause's backtrace is appended. This recovers the chained backtraces that Ruby exposes via #cause but that we were ignoring.
• Truncation now drops characters from the middle of an over-budget field instead of chopping the tail. The head usually carries the exception class/message and the tail usually carries the project-local frames; either alone is much less useful than both with a marker in between. Cuts are aligned to line boundaries so output stays readable.
• The previous formatting paths are preserved as fallbacks (legacy_format_exception_message / legacy_format_exception_backtrace) and the new code rescues StandardError to fall back if a future RSpec API shift breaks anything, so we don't regress existing behavior.

Test plan

• Verify a test that fails with a plain assertion still produces a clean failure_message and backtrace.
• Verify a test whose failure has a cause chain (e.g. an error raised from inside a rescue) shows the Caused by: block in failure_message and the cause's frames in backtrace.
• Verify a test that fails with RSpec::Core::MultipleExceptionError (test failure plus an after hook failure) reports all sub-exceptions.
• Verify an oversized backtrace gets middle-truncated with both the throw-site frames and the project frames preserved.
• Confirm bundles upload successfully end-to-end.

https://claude.ai/code/session_01BHRe4t7dxsGyoMVhLrf3CF

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.01%. Comparing base (a5dce4e) to head (d6dea80).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1085      +/-   ##
==========================================
+ Coverage   81.75%   82.01%   +0.26%     
==========================================
  Files          69       69              
  Lines       14917    14917              
==========================================
+ Hits        12195    12234      +39     
+ Misses       2722     2683      -39     

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