Migrate rules_flutter to Bazel 9

[SpencerC]

Summary

• Migrate the repository, examples, smoke tests, Gazelle module, and CI configuration to Bazel 9.
• Update Bazel module metadata, lockfiles, toolchain compatibility, and repository guidance.
• Adjust Flutter analysis and smoke-test targets for Bazel 9 behavior.

Design decisions

• Pin Bazel 9 consistently across the root repository, examples, smoke tests, and Gazelle workspace.
• Keep Bazel 9 compatibility changes localized to configuration, tooling, rule implementation, and focused tests.

Implementation decisions

• Updated .bazelrc, module files, lockfiles, CI workflows, release tooling, and documentation.
• Adapted Flutter rule definitions, extensions, private actions, analysis tests, smoke tests, and Gazelle targets.
• Updated issue and contribution templates to reflect the supported toolchain.

Links

• No linked issue or design document provided.

Reviewer guidance

• Review the Bazel 9 migration changes across the root and nested workspaces, with particular attention to module resolution, toolchain behavior, and CI/release configuration.
• Confirm the updated analysis and smoke-test targets preserve existing Flutter rule behavior.

Testing

• Not run (not requested).