Adopt telemetry providers from toolhive-core

[JAORMX]

Summary

Wave 3 adoption: delete pkg/telemetry/providers (~2,900 LOC incl. tests) and consume toolhive-core/telemetry/providers (v0.0.37, stacklok/toolhive-core#204 — a verbatim graduation of this same tree).

Changes:

• pkg/telemetry/config.go: import path swap — the option API (WithServiceName, WithOTLPEndpoint, …, NewCompositeProvider) is identical, so no call-site changes
• pkg/telemetry/providers/: deleted
• go.mod: bump toolhive-core to v0.0.37
• one stale comment path in test/e2e/dual_era_observability_test.go

What stays in toolhive (unchanged): the CRD-coupled telemetry.Config, MCP middleware, serve, propagation, attributes.

Test plan

• task build, task lint-fix — clean
• go test ./pkg/telemetry/... — pass (note: cmd/thv-operator/test-integration fails locally on missing envtest binaries, pre-existing env issue; CI has them)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.49%. Comparing base (32f75d9) to head (641702f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6146      +/-   ##
==========================================
- Coverage   72.61%   72.49%   -0.12%     
==========================================
  Files         736      729       -7     
  Lines       76340    76013     -327     
==========================================
- Hits        55431    55107     -324     
- Misses      16973    16999      +26     
+ Partials     3936     3907      -29     

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