[WIP] Upgrade MCP C# SDK to 1.2.0 for host extension

[liliankasem]

Upgrade ModelContextProtocol from 0.4.0-preview.3 to 1.2.0 and System.Net.ServerSentEvents from 10.0.0 to 10.0.5 in the host extension (Extensions.Mcp) project.

StreamableHttpTransport rewrite

SDK 1.2.0 made all StreamableHttpServerTransport properties init-only (SessionId, Stateless,
OnSessionInitialized). We can no longer set them post-construction, so all config moved into
the constructor and object initializer. We also keep a local _sessionId field because our
wrapper needs a mutable SessionId while the SDK's is immutable after init.

OnInitRequestReceived → OnSessionInitialized

The old callback was removed. The replacement has a different signature (non-nullable
InitializeRequestParams, added CancellationToken). Since it's also init-only, we pass it
through the constructor instead of the old setter pattern.

IsStateless vs Stateless

These are two separate properties on different layers — Stateless on the SDK transport
(init-only), IsStateless on our McpExtensionTransport wrapper (used by our session management).
Both must be set; not redundant.

ToolInvocationContext.Arguments: IReadOnlyDictionary → IDictionary

The SDK changed CallToolRequestParams.Arguments to IDictionary. Our property is assigned
directly from it, so types need to match. All consumers only read from it.

Serialization: must use base types

The critical discovery: ReadOnlyMemory properties (Data, Blob) must be
serialized/deserialized via base types (ContentBlock, ResourceContents), not concrete types (
ImageContentBlock, BlobResourceContents). The MCP SDK's custom polymorphic converter only
activates through the base type. Direct concrete type serialization uses STJ's default base64
converter, causing double base64 encoding. This was the root cause of all remaining test
failures.

Other type changes

• StructuredContent: JsonNode? → JsonElement? (use .GetRawText() instead of .ToJsonString())
• BlobResourceContents: use FromBytes() factory instead of setting Blob directly
• ImageContentBlock.Data / AudioContentBlock.Data: string → ReadOnlyMemory (use
  Encoding.UTF8.GetBytes(base64String))
• System.Net.ServerSentEvents: bumped to
  10.0.5 to fix NuGet downgrade error

Issue describing the changes in this PR

resolves #221

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• I have added all required tests (Unit tests, E2E tests)

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