Refactor Excel rendering and scan pipeline for improved performance

[cmendible]

Description

Refactor Excel rendering and scan pipeline for improved performance

Issue reference

We strive to have all PR being opened based on an issue, where the problem or feature have been discussed prior to implementation.

Please reference the issue this PR will close: #901

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Unit tests passing

[codecov-commenter]

Codecov Report

❌ Patch coverage is 53.67647% with 378 lines in your changes missing coverage. Please review.
✅ Project coverage is 26.11%. Comparing base (c4c45d4) to head (9370d69).

Files with missing lines	Patch %	Lines
internal/scanners/plugins/sqlesu/sqlesu.go	0.00%	87 Missing ⚠️
internal/graph/graph_scanner.go	13.55%	51 Missing ⚠️
internal/scanners/plugins/region/availability.go	0.00%	48 Missing ⚠️
internal/scanners/diagnostics_settings.go	0.00%	30 Missing ⚠️
internal/renderers/excel/excel.go	68.47%	16 Missing and 13 partials ⚠️
internal/scanners/plugins/openai/throttling.go	0.00%	24 Missing ⚠️
internal/pipeline/stage_simple.go	53.19%	17 Missing and 5 partials ⚠️
internal/az/http_client.go	19.23%	21 Missing ⚠️
internal/graph/graph.go	66.66%	8 Missing ⚠️
internal/scanners/azure_policy.go	81.81%	6 Missing and 2 partials ⚠️
... and 13 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #902      +/-   ##
==========================================
+ Coverage   21.18%   26.11%   +4.93%     
==========================================
  Files         107       96      -11     
  Lines        7568     7429     -139     
==========================================
+ Hits         1603     1940     +337     
+ Misses       5851     5374     -477     
- Partials      114      115       +1     

Flag	Coverage Δ
unittests	26.11% <53.67%> (+4.93%)	⬆️

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[cmendible]

LGTM