Expose Symbol Expression as variable in Layout Legend

[roya0045]

Description

Updated implementation.

This PR allows the user to access the expression representing a symbol rule in the Legend of a layout.

This allows the end user to perform calculation taking into acocunt the symbology.

PR aims to fix #21988 & fix #33312

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 21e325b)

[github-actions]

Tests failed for Qt 6

One or more tests failed using the build from commit 36cafc3

composer_legend_symbol_expression

composer_legend_symbol_expression

Test failed at testSymbolExpressionRender at tests/src/python/test_qgslayoutlegend.py:775

Rendered image did not match tests/testdata/control_images/composer_legend/expected_composer_legend_symbol_expression/expected_composer_legend_symbol_expression.png (found 437 pixels different)

The full test report (included comparison of rendered vs expected images) can be found here.

Further documentation on the QGIS test infrastructure can be found in the Developer's Guide.

[github-actions]

Tests failed for Qt 5

One or more tests failed using the build from commit 36cafc3

composer_legend_symbol_expression

composer_legend_symbol_expression

Test failed at testSymbolExpressionRender at tests/src/python/test_qgslayoutlegend.py:775

Rendered image did not match tests/testdata/control_images/composer_legend/expected_composer_legend_symbol_expression/expected_composer_legend_symbol_expression.png (found 278 pixels different)

The full test report (included comparison of rendered vs expected images) can be found here.

Further documentation on the QGIS test infrastructure can be found in the Developer's Guide.

[github-actions]

The QGIS project highly values your contribution and would love to see this work merged! Unfortunately this PR has not had any activity in the last 14 days and is being automatically marked as "stale". If you think this pull request should be merged, please check

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• that you have written unit tests where possible
  In case you should have any uncertainty, please leave a comment and we will be happy to help you proceed with this pull request.
  If there is no further activity on this pull request, it will be closed in a week.

[github-actions]

While we hate to see this happen, this PR has been automatically closed because it has not had any activity in the last 21 days. If this pull request should be reconsidered, please follow the guidelines in the previous comment and reopen this pull request. Or, if you have any further questions, just ask! We love to help, and if there's anything the QGIS project can do to help push this PR forward please let us know how we can assist.