propagate trace processor errors to users instead of internal logging

[svencowart]

Remove internal logging of errors in span processors and tracer provider in favor of returning errors to callers and providing an error handler callback mechanism for background operations.

Changes

• Add error_handler callback to BatchSpanProcessor for background errors (span drops, export failures during timer-based flushes)
• Return errors from shutdown_with_timeout when spans were dropped
• Remove otel_debug/otel_warn/otel_error calls from span processors
• Simplify export error handling by propagating instead of logging
• Update BatchSpanProcessor builder with with_error_handler() method
• Change dropped_spans_count to Arc for shared ownership
• Remove shutdown error logging from TracerProviderInner

This gives users explicit control over error handling rather than relying on internal SDK logging that may be missed or not actionable.

Fixes #3210

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: svencowart / name: Sven Cowart (620043d)