feat: replace hand-rolled MCP transport with rmcp SDK

[skymoore]

Summary

• Replace the custom JSON-RPC + stdio/SSE transport layer with the rmcp SDK, deleting ~300 lines of hand-rolled plumbing
• Adds spec-compliant Streamable HTTP transport with automatic Mcp-Session-Id tracking, SSE stream parsing, and content-type negotiation out of the box
• Adds configurable custom HTTP headers for authenticated remote MCP servers (Authorization, API keys, etc.)

Changes

File	Change
Cargo.toml / Cargo.lock	Add rmcp dependency with transport feature
crates/openfang-runtime/src/mcp.rs	Replace McpTransportHandle enum and manual JSON-RPC send_request/send_notification with rmcp RunningService<RoleClient>
crates/openfang-runtime/Cargo.toml	Add rmcp and http deps
crates/openfang-types/src/config.rs	Add headers field to McpServerConfigEntry
crates/openfang-kernel/src/kernel.rs	Wire headers through to McpServerConfig in all 3 connection paths
crates/openfang-extensions/src/registry.rs	Default headers for extension-generated MCP configs
crates/openfang-extensions/src/bundled.rs	Handle Http transport variant in test match
crates/openfang-api/src/routes.rs	Expose new Http transport variant in API
crates/openfang-extensions/src/lib.rs	Add Http variant to McpTransportTemplate
crates/openfang-cli/src/main.rs	Handle Http transport in CLI

Testing

• cargo build --workspace --lib - compiles clean
• cargo test --workspace - all tests pass (1700+)
• cargo clippy --workspace --all-targets -- -D warnings - zero warnings