feat(framework): Add SuperExec executor config plumbing

[msheller]

Issue

Description

SuperExec needs a small CLI/config surface so deployment-owned Helm/Kapitan
templates can select the Kubernetes executor and provide trusted internal
executor config in a file.

Related issues/PRs

Stacked on #7300.

Proposal

Explanation

This PR adds kubernetes as a SuperExec executor value, adds
--executor-config, and loads a minimal YAML kubernetes: section into a
plain internal mapping for later Kubernetes executor construction.

The config loader intentionally stays generic and permissive for F1/F2: it
parses YAML, selects the top-level section for the configured executor, and
passes that mapping through without rejecting unknown fields, converting
deployment-facing key names, or performing deep Kubernetes object validation.

Checklist

• Implement proposed change
• Write tests
• Update documentation
• Address LLM-reviewer comments, if applicable (e.g., GitHub Copilot)
• Make CI checks pass
• Ping maintainers on Slack (channel #contributions)

Any other comments?

Validated with the guarded stacked-branch refresh workflow:

• git diff --check
• py_compile on changed Python files
• dev_format_and_test.py --profile auto

[Copilot]

Pull request overview

This PR adds initial CLI/config plumbing for selecting a SuperExec executor (including a new kubernetes enum value) and optionally loading executor-specific configuration from a YAML file, with a dedicated SuperExec exit code for executor-config failures.

Changes:

• Add ExecutorType.KUBERNETES and --executor-config CLI flag, passing the parsed config into run_superexec.
• Introduce a generic YAML loader for executor config sections keyed by executor name (e.g. kubernetes:), plus initial unit tests.
• Add exit code 402 SUPEREXEC_INVALID_EXECUTOR_CONFIG and corresponding documentation page.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
framework/py/flwr/supercore/superexec/run_superexec.py	Accepts executor_config and maps executor-selection errors to exit code 402 (but currently does not apply the config).
framework/py/flwr/supercore/superexec/executor/config.py	New YAML-based executor-config loader selecting the top-level section for the chosen executor.
framework/py/flwr/supercore/superexec/executor/config_test.py	New tests covering a subset of executor-config loader error cases.
framework/py/flwr/supercore/constant.py	Adds ExecutorType.KUBERNETES.
framework/py/flwr/supercore/cli/flower_superexec.py	Adds --executor-config parsing and loads executor config before calling run_superexec.
framework/py/flwr/supercore/cli/flower_superexec_test.py	Adds CLI parsing test for kubernetes executor + config flag; asserts executor_config kwarg is present in run_superexec calls.
framework/py/flwr/common/exit/exit_code.py	Introduces exit code 402 and help text mapping.
framework/docs/source/ref-exit-codes/402.rst	Documents exit code 402 resolution steps.

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