Establish stdio JSON-RPC framing for MCP initialization

The runtime already knew how to spawn stdio MCP processes, but it still
needed transport primitives for framed JSON-RPC exchange. This change adds
minimal request/response types, line and frame helpers on the stdio wrapper,
and an initialize roundtrip helper so later MCP client slices can build on a
real transport foundation instead of raw byte plumbing.

Constraint: Keep the slice small and limited to stdio transport foundations
Constraint: Must verify framed request write and typed response parsing with a fake MCP process
Rejected: Introduce a broader MCP session layer now | would expand the slice beyond transport framing
Rejected: Leave JSON-RPC as untyped serde_json::Value only | weakens initialize roundtrip guarantees
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Preserve the camelCase MCP initialize field mapping when layering richer protocol support on top
Tested: cargo fmt --all --manifest-path rust/Cargo.toml
Tested: cargo clippy -p runtime --all-targets --manifest-path rust/Cargo.toml -- -D warnings
Tested: cargo test -p runtime --manifest-path rust/Cargo.toml
Not-tested: Integration against a real external MCP server process

rust/crates/runtime/src/lib.rs

@@ -44,6 +44,11 @@ pub use mcp_client::{
     McpClaudeAiProxyTransport, McpClientAuth, McpClientBootstrap, McpClientTransport,
     McpRemoteTransport, McpSdkTransport, McpStdioTransport,
 };
+pub use mcp_stdio::{
+    spawn_mcp_stdio_process, JsonRpcError, JsonRpcId, JsonRpcRequest, JsonRpcResponse,
+    McpInitializeClientInfo, McpInitializeParams, McpInitializeResult, McpInitializeServerInfo,
+    McpStdioProcess,
+};
 pub use oauth::{
     code_challenge_s256, generate_pkce_pair, generate_state, loopback_redirect_uri,
     OAuthAuthorizationRequest, OAuthRefreshRequest, OAuthTokenExchangeRequest, OAuthTokenSet,