snip source correctly for dep tracker

[adhami3310]

fixes #5580

[greptile-apps]

Greptile Summary

This PR addresses a critical bug in Reflex's dependency tracking system, specifically in how source code snippets are handled during variable analysis. The change modifies the _eval_var method in reflex/vars/dep_tracking.py to correctly extract source code when evaluating variable dependencies.

The fix specifically adjusts the string slicing logic to properly handle multi-line source code snippets. When extracting variable names from source code, it now correctly joins the source lines while preserving the complete variable names, addressing a truncation issue that was causing variables like 'loaded_experiment' to be incorrectly identified as 'loaded_experimen'.

This change is fundamental to Reflex's state management system, ensuring accurate dependency tracking and state variable access through get_var_value.

PR Description Notes:

• The PR description could be more detailed, explaining the problem and solution more thoroughly

Confidence score: 5/5

1. This PR is very safe to merge as it fixes a clear bug with a straightforward solution
2. The changes are minimal, focused, and the logic correction is evident - improper string slicing is replaced with correct concatenation
3. Key files needing attention: reflex/vars/dep_tracking.py

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[codspeed-hq]

CodSpeed Performance Report

Merging #5613 will not alter performance

_{Comparing snip-source-correctly-for-dep-tracker (7e5c49e) with main (b052299)}

Summary

✅ 8 untouched benchmarks

[masenf]

can we get a unit test added for this

[adhami3310]

can we get a unit test added for this

the code is a bit weird, but i will try

[masenf]

nice