chore: fix spelling errors in backend.cc, reduce.t.cc, and exponent_aggregates_computation.cc

[vipocenka]

1. backend.cc:

   • Fixed "varaible" → "variable" in a log message.

2. exponent_aggregates_computation.cc:

   • Corrected multiple instances of "aggegate_term" → "aggregate_term" in function definitions and calls.

3. reduce.t.cc:

   • Fixed "begginning" → "beginning" in multiple comments describing memory operations.

Are These Changes Tested?

These updates only correct spelling errors in comments and function names. No functional changes were made, so existing tests should continue to pass without modifications.

Type of Change

• Bug fix
• New feature
• Documentation update
• Enhancement

Checklist

• Self-reviewed the changes.
• Opened or linked an existing issue if applicable.

Impact

Impact: Low. These changes are limited to spelling corrections in comments and function names, with no effect on functionality.

[jacobtrombetta]

Running the code format script should fix the failing CI test. See here: https://github.com/spaceandtimelabs/blitzar?tab=readme-ov-file#code-format. Tell me if you don't have nix installed and I can help out.

[rnburn]

I copied these changes over to #246 and fixed the formatting. Thank you.