fix(server): reject overlapping historical gRPC block ranges

[songgaoye]

Description

Closes: #XXXX

Historical gRPC routing iterates over a map with random iteration order.
Overlapping block ranges cause nondeterministic backend selection across
runs. Added validation in GetConfig to detect and reject overlapping
ranges at parse time with clear error messages.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.24%. Comparing base (7af05f5) to head (17aeed3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25807      +/-   ##
==========================================
+ Coverage   68.16%   68.24%   +0.08%     
==========================================
  Files         910      894      -16     
  Lines       59051    58158     -893     
==========================================
- Hits        40250    39688     -562     
+ Misses      18801    18470     -331     

Files with missing lines	Coverage Δ
server/config/config.go	93.68% <100.00%> (+0.58%)	⬆️

... and 18 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[aljo242]

ci failing