Add proposal: Upstream MCP Registry (server.json) integration for ToolHive run flow

[JAORMX]

This PR adds a proposal for integrating the upstream MCP Registry (server.json) with ToolHive’s run flow.

Overview

• Keep thv run inputs unchanged (by name, image, protocol, remote URL)
• Resolve names via the configured registry; when upstream (URL) registry is configured, fetch server.json and convert to ToolHive metadata
• Initial package types supported: oci (direct image), npm (npx://), pypi (uvx://)
• Minimal code change: in the registry path, detect protocol strings produced by conversion and synthesize images via the existing protocol builder (IsImageProtocolScheme + HandleProtocolScheme) before verification/pull
• Align internal upstream model/converter to official schema fields (registry_type, identifier, version, transport)
• No changes to builder/runner/middleware; other ecosystems can be added later

Document: docs/proposals/upstream-mcp-registry-format-support.md

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.93%. Comparing base (24a2f37) to head (1ba2858).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1918      +/-   ##
==========================================
+ Coverage   46.90%   46.93%   +0.02%     
==========================================
  Files         220      220              
  Lines       27319    27319              
==========================================
+ Hits        12814    12822       +8     
+ Misses      13530    13515      -15     
- Partials      975      982       +7     

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[coveralls]

coverage: 44.281% (+0.04%) from 44.242%
when pulling 1ba2858 on proposal-mcp-registry-upstream-support
into 24a2f37 on main.