Remove step-scoped data from extension contributors (#34734)

## What changed

- Remove the step-scoped `ExtensionData` argument from context, turn-input, and tool contributors.
- Pass the host's optional `McpResourceClient` through `ThreadStartInput` so extensions can retain session capabilities explicitly.
- Keep the MCP resource client in skills-owned session state for catalog loading and skill tools.

GitOrigin-RevId: bafa77bcd998aff408d6a396c5fd9ac268c4cce4

codex-rs/ext/extension-api/Cargo.toml

@@ -17,6 +17,7 @@ workspace = true
 codex-config = { workspace = true }
 codex-context-fragments = { workspace = true }
 codex-exec-server-protocol = { workspace = true }
+codex-mcp = { workspace = true }
 codex-protocol = { workspace = true }
 codex-tools = { workspace = true }
 codex-utils-absolute-path = { workspace = true }