Add a helper to create config flow `step_id` names from functions

[puddly]

Proposed change

During development, I've run into a few situations where the config flow step ID string does not refer to a real step. This is only caught at runtime, as opposed to by the type checker.

This PR introduces FlowHandler.get_step_id to derive the step_id directly from a reference to the function:

 return self.async_show_form(
-    step_id="first",
+    step_id=self.get_step_id(self.async_step_first),
     data_schema=vol.Schema([str]),
 )

Any mistakes are caught by mypy directly:

mypy.....................................................................Failed
- hook id: mypy
- exit code: 1

homeassistant/components/zha/config_flow.py:779: error: "ZhaOptionsFlowHandler" has no attribute "async_step_intent_migrates"; maybe "async_step_intent_migrate" or "async_step_intent_reconfigure"?  [attr-defined]
Found 1 error in 1 file (checked 1 source file)

An alternative implementation would be to add a step= kwarg to all of the FlowHandler functions and pass the step function in directly.

Type of change

• [ ] Dependency upgrade
• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New integration (thank you!)
• [ ] New feature (which adds functionality to an existing integration)
• [ ] Deprecation (breaking change to happen in the future)
• [ ] Breaking change (fix/feature causing existing functionality to break)
• [x] Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:
• Link to developer documentation pull request:
• Link to frontend pull request:

Checklist

• [x] The code change is tested and works locally.
• [x] Local tests pass. Your PR cannot be merged unless tests pass
• [x] There is no commented out code in this PR.
• [x] I have followed the development checklist
• [x] I have followed the perfect PR recommendations
• [x] The code has been formatted using Ruff (ruff format homeassistant tests)
• [x] Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• [ ] Documentation added/updated for www.home-assistant.io

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• [ ] The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• [ ] New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• [ ] For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.

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[puddly]

CI failure is in Roku and unrelated to this PR.

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