Fix OTel insecure and prometheus settings fallback to global config

[danbarr]

Fixes #1899

The insecure and enable-prometheus-metrics-path configs were not falling back to the global config file settings when the CLI flags aren't provided as overrides. This caused ToolHive to:

• Attempt HTTPS connections even when thv config otel set-insecure true was configured
• Not expose the Prometheus endoing even when thv config otel set-enable-prometheus-metrics-path true was configured

Changes:

• Added fallback logic for insecure and enable-prometheus-metrics-path settings in getTelemetryFromFlags()
• Enhanced tests to verify fallback behavior of the two new flags

NOTE: this issue also affects the metrics-enabled and tracing-enabled flags that were also added in #1785, but since these default to true, I had trouble solving for them in the same way, since unset and explicit false config settings are currently indistinguishable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.39%. Comparing base (f917be9) to head (e0dfbec).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1900      +/-   ##
==========================================
+ Coverage   45.77%   46.39%   +0.62%     
==========================================
  Files         203      206       +3     
  Lines       25979    26145     +166     
==========================================
+ Hits        11891    12131     +240     
+ Misses      13173    13094      -79     
- Partials      915      920       +5     

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