Feat: Allow negative stack_trace_limit in StdlibFormatter

[choieastsea]

Overview

• Accept negative stack_trace_limit value and pass through to traceback.format_tb() since it supports negative limits for collecting last frames.
• Update stdlib formatter tests to cover both positive and negative limits.

Additional Context

• Aligns with Python traceback behavior (limit < 0 means last N frames)

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🔍 Preview links for changed docs

• docs/reference/installation.md

[choieastsea]

Hi, I’m using this library for production and would like to capture only the most recent error frames.

Since stdlibformatter passes the value through to Python’s traceback, which already supports negative limits to mean “last N frames,” it felt reasonable to allow a negative stack_trace_limit here as well. Please let me know if you’d prefer a different approach.