Adopt core MCP semconv bucket preset in telemetry

[JAORMX]

Summary

Source pkg/telemetry.MCPHistogramBuckets from toolhive-core/telemetry/metrics.BucketsMCPSemconv() instead of a locally-defined literal. Callers (pkg/vmcp/..., pkg/ratelimit) are untouched — the exported var keeps the same name and value set.

Bumps toolhive-core to v0.0.36, which contains the preset (stacklok/toolhive-core#202, merged).

Test plan

• task build, task lint-fix — clean
• go test ./pkg/telemetry/... ./pkg/vmcp/server/sessionmanager/... ./pkg/vmcp/internal/backendtelemetry/... ./pkg/vmcp/core/... ./pkg/ratelimit/... — all pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.58%. Comparing base (3d44435) to head (5141c59).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6144      +/-   ##
==========================================
- Coverage   72.63%   72.58%   -0.06%     
==========================================
  Files         736      736              
  Lines       76355    76355              
==========================================
- Hits        55463    55423      -40     
- Misses      16955    17009      +54     
+ Partials     3937     3923      -14     

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