fix(telemetry): warn instead of crashing when OTLP endpoint has no enabled exporters

[peppescg]

Summary

• When an OTLP endpoint is configured (thv config otel set-endpoint) but both tracing and metrics are disabled, thv serve crashes with a fatal validation error (exit code 1)
• This causes ToolHive Studio to show a "Health check failed" error with no clear indication of the root cause
• Instead of crashing, we now enable tracing by default when an endpoint is configured but no exporters are enabled, so the endpoint is actually used rather than silently ignored

Test plan

• Configured an OTLP endpoint (thv config otel set-endpoint localhost:4317) with tracing/metrics not enabled
• Verified thv serve starts with a warning and tracing enabled by default, instead of crashing
• Verified existing telemetry tests pass (go test ./pkg/telemetry/...)
• Verify normal OTLP flow still works when tracing or metrics are explicitly enabled

Fixes #4647

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.82%. Comparing base (4529b8e) to head (738dc2a).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/telemetry/serve.go	0.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4648      +/-   ##
==========================================
- Coverage   68.85%   68.82%   -0.03%     
==========================================
  Files         506      506              
  Lines       52449    52460      +11     
==========================================
- Hits        36115    36108       -7     
- Misses      13542    13561      +19     
+ Partials     2792     2791       -1     

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