agent: Store if context server should be enabled/disabled in the settings (#32994)

- [x] Show disabled MCP servers in the list so you can enable them again
- [x] If MCP is not present in the settings, add it

Closes #ISSUE

Release Notes:

- agent: Allow to enable/disable context servers permanently in the
agent configuration view

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

crates/extension_host/src/wasm_host/wit/since_v0_6_0.rs

@@ -949,6 +949,7 @@ impl ExtensionImports for WasmState {
 
                         match settings {
                             project::project_settings::ContextServerSettings::Custom {
+                                enabled: _,
                                 command,
                             } => Ok(serde_json::to_string(&settings::ContextServerSettings {
                                 command: Some(settings::CommandSettings {
@@ -959,6 +960,7 @@ impl ExtensionImports for WasmState {
                                 settings: None,
                             })?),
                             project::project_settings::ContextServerSettings::Extension {
+                                enabled: _,
                                 settings,
                             } => Ok(serde_json::to_string(&settings::ContextServerSettings {
                                 command: None,