Fix XRD block legend display issues

[BenjaminCharmes]

Closes #1485

• Remove legend_label from circle glyphs to prevent point/line desynchronization when toggling legend visibility
• Add sequential numbering to shortened legend labels that would otherwise be identical after truncation (e.g., "CIF_00...xrdml [01]", "CIF_00...xrdml [02]")

[codecov]

Codecov Report

❌ Patch coverage is 78.87324% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.04%. Comparing base (020cc77) to head (29e518a).

Files with missing lines	Patch %	Lines
pydatalab/src/pydatalab/utils.py	75.80%	30 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1489      +/-   ##
==========================================
- Coverage   79.12%   79.04%   -0.08%     
==========================================
  Files          71       71              
  Lines        5413     5550     +137     
==========================================
+ Hits         4283     4387     +104     
- Misses       1130     1163      +33     

Files with missing lines	Coverage Δ
pydatalab/src/pydatalab/apps/xrd/blocks.py	90.27% <ø> (ø)
pydatalab/src/pydatalab/bokeh_plots.py	80.36% <100.00%> (+0.81%)	⬆️
pydatalab/src/pydatalab/utils.py	76.39% <75.80%> (-10.09%)	⬇️

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[cypress]

datalab    Run #4371

Run Properties:   Passed #4371  •  59a895c49b ℹ️: Merge 29e518acea0e1a80fa90487ae32708696b668b57 into 020cc770e22581fddcfdfdba2feb...

Project	datalab
Branch Review	bc/xrd-fix
Run status	Passed #4371
Run duration	09m 09s
Commit	59a895c49b ℹ️: Merge 29e518acea0e1a80fa90487ae32708696b668b57 into 020cc770e22581fddcfdfdba2feb...
Committer	Ben Charmes
View all properties for this run ↗︎

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Test results
Failures	0
Flaky	0
Pending	0
Skipped	0
Passing	458
View all changes introduced in this branch ↗︎

[ml-evs]

Legend behaviour works fine (could merge this separately) but think the label generation still needs some work