Refactor ExecutableCoordinator interface based on dev-list design discussion

[jason810496]

Background

The current ExecutableCoordinator
(providers/sdk/executable/src/airflow/providers/sdk/executable/coordinator.py)
is the shared Python-side launcher for any native-executable SDK (Go, Rust,
C++, Zig, ...) that produces an AFBNDL01-trailered bundle. It currently
exposes can_handle_dag_file, get_code_from_file, dag_parsing_cmd,
and task_execution_cmd and handles both the pure-executable Dag path
and the Python @task.stub Dag path.

A design discussion is in flight on the dev list about how this interface
should evolve as more lang-SDKs adopt it. The decision is not yet final;
this issue exists to track the eventual refactor so the deferred work has
a stable URL.

What needs to happen

1. Reach consensus on the dev-list thread for the new interface shape.
2. Apply the agreed-upon refactor to ExecutableCoordinator.
3. Update the bundle-spec doc (providers/sdk/executable/docs/bundle-spec.rst)
   to match.
4. Update the Go SDK's bundlev1server.Serve coordinator-mode dispatcher
   (ADR 0003) only if the interface change requires a wire-level adjustment.
5. Migrate any in-tree subclasses (Java provider, future Go provider) at
   the same time.

Acceptance criteria

• ExecutableCoordinator interface matches the dev-list outcome.
• ADR 0003 / 0004 in go-sdk/adr/ are updated to reflect the new shape.
• E2E tests still pass for both the pure-executable and Python-stub paths.

Context

• Design discussion: https://lists.apache.org/thread/gjot4bxj9kygj2fk76kx6tyg8s4hr057
• Related Java-side interface work: Improve BaseCoordinator interface to support pluggable communication channels #66838 (pluggable BaseCoordinator).
• Recent fix that motivates the cleanup: commit 8612c4574f2 ("Fix command
  resolution in ExecutableCoordinator for improved task execution").

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