feat: add RubyGems (rubygems.org) as a package registry type - #1859

Motivation and Context

Ruby MCP servers published to rubygems.org cannot be registered through their native package registry today; the only options are an OCI image or an MCPB download. This adds registryType "rubygems", following docs/contributing/add-package-registry.md and mirroring the cargo work (#1207).

Design decisions:

• Ownership verification reads the gemspec metadata key "mcp_name" from the version-specific rubygems.org API (/api/v2/rubygems/{name}/versions/{version}.json), which covers existence, version, and ownership in a single call. This mirrors the npm validator's manifest-field approach (mcpName in package.json) rather than the README-token approach used for PyPI/NuGet/cargo: rubygems.org has no README API, and gemspec metadata is a first-class, author-controlled field.
• The platform is left unspecified in that call so that versions published only as platform-specific builds can be registered. rubygems.org then chooses which build to return, so authors must set mcp_name in every platform build.
• A 404 from the version endpoint is disambiguated with a HEAD probe of /api/v1/versions/{name}.json (gem missing vs version missing vs transient), following the npm validator. Yanked versions 404 on the version endpoint (verified against mimemagic 0.3.0); a 200 carrying yanked=true is still rejected defensively.
• The documented client invocation is gem exec --silent --conservative --gem NAME -v VERSION COMMAND: --silent keeps install progress off stdout (required for the stdio transport), and --conservative runs offline when the version is already installed. gem exec ships with RubyGems 3.4.8+ (Ruby 3.2.2+), so runtimeHint "gem" is set, unlike cargo which has no single-shot runner.

Closes #1582.

How Has This Been Tested?

• Validator tests are hermetic httptest mocks, including the positive path, plus live anchors against rubygems.org negative paths (missing mcp_name, missing gem, yanked version).
• make validate, make test-unit, and make test-integration pass, and golangci-lint v2.14.0 reports 0 issues.
• The documented invocation was run on RubyGems 4.0.3 from an empty GEM_HOME against an existing MCP server gem (debug-mcp 0.4.0): the first line on stdout is the JSON-RPC initialize response. Without --silent the first line is install output instead. With --conservative an installed version starts with the network blocked; without it the start fails. A -- before COMMAND makes gem exec fail, so the documentation tells clients not to insert one.
• Not tested: a publish against a real gem carrying mcp_name. I know of no such gem yet (none among the latest versions of 489 MCP-related gems on rubygems.org), so a live positive-path anchor will be added once one is published.

Breaking Changes

None. The change is additive: existing registry types and their validation are untouched.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed