fix(js/genkit): Explicitly allow null types in promptDir

[7hokerz]

Description here... Help the reviewer by:

• Runtime already treats promptDir: null as an opt-out for automatic ./prompts loading.
• The problem is that while runtime logic seems to treat null values ​​specially and allow them, type does not.
• This change aligns the type definition with existing behavior and adds coverage for the null opt-out case.

In genkit.ts (972-978 line)

private configure() {
...
if (this.options.promptDir !== null) {
      loadPromptFolder(
        this.registry,
        this.options.promptDir ?? './prompts',
        ''
      );
    }
...

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Code Review

This pull request introduces the ability to disable automatic prompt loading by allowing promptDir to be set to null in GenkitOptions. However, feedback indicates that the implementation of the prompt() method still defaults to the standard directory when null is provided, meaning the opt-out behavior is not yet fully functional. Furthermore, the added test case needs to be more robust to ensure it correctly verifies that file-based loading is disabled even when prompts exist in the default directory.

[MichaelDoyle]

Looping in @ifielker and @pavelgj since they have more context than I do.

[pavelgj]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces the ability to disable automatic prompt loading by setting the promptDir option to null. Changes include updating type definitions in prompt.ts and genkit.ts, adjusting the logic in the Genkit class to respect the null value instead of defaulting to the ./prompts directory, and adding a unit test to verify that prompts are not loaded when promptDir is null. I have no feedback to provide.