Fix resource relay to preserve text/blob distinction

[JAORMX]

Summary

The vMCP relay was concatenating all resource contents into a single []byte with one MimeType, losing the text/blob distinction, per-item URIs, and per-item MIME types when a resource returned multiple content items. This meant multi-content resources and blob resources were degraded through the relay.

This changes ResourceReadResult from Contents []byte + MimeType string to Contents []ResourceContent, preserving each item's type (text vs blob), URI, MIME type, and data independently.

• Add ResourceContent type with Text, Data, URI, MimeType fields to vmcp.types
• Replace lossy ConcatenateResourceContents with ConvertMCPResourceContents / ToMCPResourceContents round-trip converters
• Update handler factory to relay structured resource contents instead of reconstructing from concatenated bytes
• Remove the now-unused concatenation helper and its tests
• Update all tests to assert on structured resource content

Type of change

• Bug fix

Test plan

• Unit tests (task test)
• Linting (task lint-fix)

Changes

File	Change
pkg/vmcp/types.go	Add ResourceContent type, update ResourceReadResult
pkg/vmcp/conversion/content.go	Add ConvertMCPResourceContents / ToMCPResourceContents, remove ConcatenateResourceContents
pkg/vmcp/conversion/conversion_test.go	Add round-trip tests for resource contents
pkg/vmcp/client/client.go	Use structured conversion instead of concatenation
pkg/vmcp/server/adapter/handler_factory.go	Relay structured contents directly
pkg/vmcp/server/adapter/handler_factory_test.go	Update to use structured resource content
pkg/vmcp/server/sessionmanager/session_manager.go	Update resource content handling
pkg/vmcp/session/internal/backend/mcp_session.go	Use structured conversion

Does this introduce a user-facing change?

Resource responses from virtual MCP servers now preserve individual content items with their text/blob types, URIs, and MIME types, instead of concatenating everything into a single byte slice.

Special notes for reviewers

This is one of three related PRs fixing vMCP relay fidelity:

• fix-prompt-messages: prompt message structure
• This PR: resource text/blob distinction and per-item metadata
• fix-content-annotations: per-content annotations (audience, priority, lastModified)

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