Update span-stacktrace module to follow style guide

[Copilot]

Updates the span-stacktrace module to comply with the OpenTelemetry Java Contrib style guide requirements by making minimal, targeted changes:

Changes Made

1. Added final keyword to public non-internal classes:

   • StackTraceSpanProcessor - now declared as public final class
   • StackTraceAutoConfig - now declared as public final class

2. Added static imports for commonly used constants:

   • Static imports for Level.FINE and Level.SEVERE in StackTraceAutoConfig
   • Removed redundant Level import and updated usage to use static imports

These changes align with the style guide requirements that:

• Public non-internal classes should be declared final where possible
• Static imports should be used for commonly used constants like logging levels

All existing functionality is preserved and tests continue to pass. The changes are minimal and surgical, focusing only on style guide compliance without affecting behavior.

Fixes #413.

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