fix: address 2025-11-25 conformance audit findings

[DaleSeo]

Motivation and Context

A conformance audit of rmcp against the MCP 2025-11-25 spec (schema cross-check plus the official conformance suite v0.1.16) surfaced four small, independent fixes:

• OperationProcessor interpreted ttl as seconds and applied it via Duration::from_secs, but the spec defines ttl in milliseconds — a spec-conformant ttl: 60000 became a ~16.7h deadline.
• ResourceContents::text() / ContentBlock::embedded_text defaulted mimeType to "text", which is not a valid MIME type. Now text/plain.
• Refresh-token requests omitted the RFC 8707 resource parameter, which the spec requires on all token requests. Authorization and code-exchange requests already included it.
• The in-tree conformance server declared prompt arguments name/style while the official runner sends arg1/arg2, making prompts-get-with-args the only failing server scenario.

Also adds a CI workflow that runs the official server conformance suite on PRs (related: #684).

How Has This Been Tested?

Official conformance suite v0.1.16 against the in-tree server: 40/40 (was 39/40).

Breaking Changes

None

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