python-sdk: add CLI 1.0 client init

CLI 1.0 calls initClient when a workspace adds a managed client. The Python
SDK init-contract work lives in the stacked prepare-cli-1.0-init-contract
branch, but that branch does not add the client hook.

Add initClient with the CLI 1.0 signature, including the dev flag. It returns
an empty Changeset on purpose: the engine records the client and owns generated
client files through GeneratedContextChangeset.

Add an e2e check that calls initClient and verifies the SDK returns no file
changes.

This keeps the PR narrowly scoped on top of #2. It only adds the client init
hook and does not duplicate the targetRuntime or initModule changes from the
stacked base.

Signed-off-by: Guillaume de Rouville <guillaume@dagger.io>

Author: grouville

.dagger/modules/e2e/main.dang

@@ -156,6 +156,18 @@ type E2e {
     null
   }
 
+  """
+  initClient should not materialize files itself. The engine records the client
+  and owns the generated context changeset.
+  """
+  pub initClientCheck(ws: Workspace!): Void @check {
+    let changes = pythonSdk.initClient(ws, path: outputRoot + "/client", module: generateModulePath, dev: true)
+
+    assert(changes.isEmpty, "initClient should return an empty SDK changeset")
+
+    null
+  }
+
   """
   config.get should reflect pyproject.toml and report unset values as null
   rather than guessing, and config.set should edit only pyproject.toml.

python-sdk.dang

@@ -145,6 +145,16 @@ type PythonSdk {
     }
   }
 
+  """
+  Generate a typed Python client for `module` at `path`.
+
+  The engine records the managed client in workspace config before calling
+  this function. The engine owns materializing the generated client files.
+  """
+  pub initClient(ws: Workspace!, path: String!, module: String!, dev: Boolean! = false): Changeset! {
+    polyfill.workspace(ws).fork.changes
+  }
+
   """
   Render a Python template with the requested module name.
   """