Make observation localization radius defaulted on config creation

[xjules]

Issue
Resolves #13273

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Add backport label to latest release (format: 'backport release-branch-name')

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.98%. Comparing base (9a43f3a) to head (6f1f7ae).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13289      +/-   ##
==========================================
- Coverage   90.00%   89.98%   -0.03%     
==========================================
  Files         457      457              
  Lines       31581    31585       +4     
==========================================
- Hits        28426    28422       -4     
- Misses       3155     3163       +8     

Flag	Coverage Δ
cli-tests	37.47% <100.00%> (-0.01%)	⬇️
fuzz	44.52% <100.00%> (-0.01%)	⬇️
gui-tests	67.16% <100.00%> (-0.03%)	⬇️
performance-and-unit-tests	77.93% <100.00%> (-0.01%)	⬇️
test	46.20% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/ert/config/_create_observation_dataframes.py	90.90% <ø> (-0.12%)	⬇️
src/ert/config/_observations.py	95.48% <100.00%> (+0.01%)	⬆️

... and 3 files with indirect coverage changes

[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks

---

_{Comparing xjules:default_radius (6f1f7ae) with main (7c245db)}