fix(SyntheticTimeSeries): Make synthetic time series more robust with special characters

[MortGron]

Description

By mistake, synthetic time series queries will fail when re.escape(var) != var for some variable var in a query expression before variable replacement. However, they do not fail when variables contain characters like \.

Even though this fixes a bug, a long term solution to make variable names safer is #2762

Checklist:

• Tests added/updated.
• Documentation updated. Documentation is generated from docstrings - these must be updated according to your change.
  If a new method has been added it should be referenced in cognite.rst in order to generate docs based on its docstring.
• The PR title follows the Conventional Commit spec.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.08%. Comparing base (51d717d) to head (e2cb6e8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2766      +/-   ##
==========================================
- Coverage   93.09%   93.08%   -0.02%     
==========================================
  Files         516      516              
  Lines       53590    53597       +7     
==========================================
+ Hits        49889    49890       +1     
- Misses       3701     3707       +6     

Files with missing lines	Coverage Δ
cognite/client/_api/synthetic_time_series.py	96.00% <100.00%> (+0.03%)	⬆️
cognite/client/_sync_api/synthetic_time_series.py	100.00% <ø> (ø)
.../tests_unit/test_api/test_synthetic_time_series.py	97.70% <100.00%> (+0.10%)	⬆️

... and 6 files with indirect coverage changes

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[gemini-code-assist]

Code Review

This pull request updates the synthetic time series expression builder to escape variable names during regex compilation rather than when storing them in the substitution dictionary, and updates the unit tests accordingly. The reviewer suggests sorting the substitution keys by length in descending order before compiling the regex pattern to prevent shorter prefixes from matching first when they contain non-word characters.

[haakonvt]

🦄 Preferably add test cases in a parametrize, but not a hard requirement