test: refactor e2e test suite

[lukasmetzner]

• Use t.Cleanup for teardown
• context.Background() → t.Context() in tests/helpers
• %w error wrapping
• Standardize on testify
• Exponential backoff (1s → 30s cap) for CreateService + WaitForHTTPAvailable

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.62%. Comparing base (a9d4851) to head (8270299).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1187      +/-   ##
==========================================
+ Coverage   66.05%   69.62%   +3.57%     
==========================================
  Files          24       24              
  Lines        2657     2657              
==========================================
+ Hits         1755     1850      +95     
+ Misses        735      634     -101     
- Partials      167      173       +6     

Flag	Coverage Δ
e2e	43.79% <ø> (?)
unit	66.05% <ø> (ø)

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