fix(config): migrate another telemetry config

[jerm-dro]

Summary

Fixes #3267

This PR adds another migration to finish migrating the other instance of the telemetry.Config in the runtime config.

Details

The original fix for the bug only migrated the telemetry_config field in the runtime config. However, it's also possible for telemetry.Config to exist in the runtimeConfig.middleware_configs[i].parameters.config as seen below:

{
  "middleware_configs": [
    {
      "type": "telemetry",
      "parameters": {
        "config": {
          "endpoint": "",
          "serviceName": "toolhive-mcp-proxy",
          "serviceVersion": "v0.6.13",
          "tracingEnabled": true,
          "metricsEnabled": true,
          "samplingRate": 1,
          "headers": {},
          "insecure": true,
          "enablePrometheusMetricsPath": true,
          "environmentVariables": [
            "USER",
            "HOST"
          ]
        },
        "server_name": "mermaid",
        "transport": "streamable-http"
      }
    }
  ]
}

How was this missed?
The parameters field is an untyped json blob.

Changes

• refactor the telemetry migration so it runs against both telemetry_config and the middleware_configs.
• introduce another migration to ensure the middleware_configs is migrated in the event users have already upgraded
• add some tests to verify the migration of middleware_configs according to the structure above.

[codecov]

Codecov Report

❌ Patch coverage is 53.33333% with 42 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.75%. Comparing base (bc23d68) to head (808709a).
⚠️ Report is 38 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/migration/telemetry_config.go	59.15%	23 Missing and 6 partials ⚠️
pkg/migration/middleware_telemetry.go	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3328      +/-   ##
==========================================
+ Coverage   64.62%   64.75%   +0.13%     
==========================================
  Files         369      373       +4     
  Lines       35953    36344     +391     
==========================================
+ Hits        23236    23536     +300     
- Misses      10890    10955      +65     
- Partials     1827     1853      +26     

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[jhrozek]

Thanks for fixing the middleware telemetry config migration! I have a few questions about the implementation to make sure I understand the intended behavior.