ExecutableCoordinator: rename bundles_folder, accept list[str], switch coordinator config to dict-keyed shape #66940

Background

ExecutableCoordinator currently reads its bundle directory from
[executable] bundles_folder:

# providers/sdk/executable/src/airflow/providers/sdk/executable/coordinator.py:123
bundles_folder = conf.get("executable", "bundles_folder", fallback=None)

The Java side has just hit the same problem and is tracking the same
refactor under #66836:

1. bundles_folder is easy to confuse with Airflow's Dag bundle concept.
2. Single string is too narrow - users need multiple bundle directories.
3. Top-level coordinators config should be a dict keyed by coordinator
   name to enable lazy init and clean per-coordinator kwargs.

#66451 already landed the dict-keyed shape for JavaCoordinator; this
issue ports the same shape onto ExecutableCoordinator.

What needs to happen

1. Rename [executable] bundles_folder to a clearer key (e.g.
   executables_root or bundles_root). Bake the rename into
   providers/sdk/executable/provider.yaml and the docs.
2. Accept list[str]: scan every directory in order.
3. Allow per-coordinator kwargs (executable_path, env,
   working_dir) under [sdk] coordinators keyed by name, matching
   the shape Improve coordinator construction to support language runtime configs #66451 settled on for Java.

Acceptance criteria

• New config shape is the documented form; old bundles_folder is
  removed (or aliased with a deprecation warning if a release has
  already been cut).
• Multiple bundle directories scan correctly.
• E2E tests cover the new shape.

Context

• Java parallels: JavaCoordinator: Rename bundles_folder, accept list[str], and switch coordinator config to dict #66836 (rename + list[str] + dict-keyed),
  Improve coordinator construction to support language runtime configs #66451 (Done - dict-keyed coordinator construction).
• Same dev-list thread covers the cross-SDK consistency story:
  https://lists.apache.org/thread/gjot4bxj9kygj2fk76kx6tyg8s4hr057

Drafted-by: Claude Code (Opus 4.7) (no human review before posting)