otlploghttp: normalize global endpoint path joining

[RedZapdos123]

Description:

Issue #8863 reported that otlploghttp could build a double-slash request path when the global OTEL_EXPORTER_OTLP_ENDPOINT base URL ended with a trailing slash.

convPath currently appends "/v1/logs" directly to u.Path, so a base endpoint like http://host/prefix/ resolves to /prefix//v1/logs.

This PR normalizes the global OTLP endpoint path join for otlploghttp and adds a regression test covering a trailing-slash base endpoint.

It does not change WithEndpointURL behavior.

Closes #8863.

Verifications:

• cd exporters/otlp/otlplog/otlploghttp && go test . -run TestNewConfig -count=1 -v
• cd exporters/otlp/otlplog/otlploghttp && go test . -count=1
• cd exporters/otlp/otlplog/otlploghttp && go test ./... -count=1

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.5%. Comparing base (d093c45) to head (6b7dc58).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #8864     +/-   ##
=======================================
- Coverage   88.5%   88.5%   -0.1%     
=======================================
  Files        331     331             
  Lines      21013   21013             
=======================================
- Hits       18603   18602      -1     
- Misses      2410    2411      +1     

Files with missing lines	Coverage Δ
exporters/otlp/otlplog/otlploghttp/config.go	96.2% <100.0%> (ø)

... and 3 files with indirect coverage changes

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[RedZapdos123]

Resolved the merge conflict.