python-sdk: add CLI 1.0 client init

[grouville]

Why

This PR is stacked on #2 (prepare-cli-1.0-init-contract). #2 carries the Python SDK module init contract work. This PR keeps the next piece narrow: add the CLI 1.0 initClient hook that the engine calls when a workspace adds a managed client, add e2e coverage for that hook, and load the e2e module through workspace config.

Dogfood workspace checks also exposed that Python still pinned an older sdk-sdk/polyfill. Its helper binaries were built against dagger.io/dagger v0.20.6 and queried loadWorkspaceFromID, which fails against the CLI 1.0 dev engine schema. This PR bumps the polyfill pin/lock to the same commit used by the TypeScript SDK.

What Changed

1. Add initClient.

   • Signature matches the CLI 1.0 SDK contract, including dev.
   • Implementation returns an empty Changeset.
   • The engine already records managed clients and owns generated client materialization via the generated context changeset.

2. Add e2e coverage.

   • The e2e module calls pythonSdk.initClient(...).
   • It asserts that the SDK returns an empty changeset.

3. Load e2e through workspace config.

   • dagger.toml installs only .dagger/modules/e2e as e2e.
   • The Python SDK root module is not added as a workspace module.
   • The e2e module engine pin moves to v0.21.4, matching the TypeScript SDK e2e module.

4. Refresh the polyfill pin used by workspace helper commands.

   • Bumps sdk-sdk/polyfill from 09da957... to d1532df....
   • Updates .dagger/lock to the same commit.
   • Fixes workspace-module-generate and workspace-snapshot against the CLI 1.0 dev engine schema.

What This Does Not Do

This PR does not duplicate the targetRuntime or initModule changes from the stacked base branch. It should be reviewed as the client-init and workspace-load delta on top of #2.

Validation

Run with the local CLI 1.0 dev engine:

/home/dagger/dagger/hack/with-dev dagger --progress=plain session --load-workspace-modules --label dagger.io/sdk.name:go --label dagger.io/sdk.version:n/a
/home/dagger/dagger/hack/with-dev dagger --progress=plain call e-2-e init-client-check
/home/dagger/dagger/hack/with-dev dagger --progress=plain call e-2-e generate-all-check
/home/dagger/dagger/hack/with-dev dagger --progress=plain call e-2-e config-check