fix: don't inherit parent Form props in a nested Form

[UditDewan]

Closes #10740

A RAC Form both provides and consumes FormContext via useContextProps, so a Form rendered inside another Form in the React tree (e.g. an independent form in a Modal) merged in the outer form's props: chained onSubmit, duplicated id/className, and inherited validationErrors.

Intent: a nested Form should be independent, while a FormContext provided by the user (not by a Form) should keep working as it does today.

Implementation: following the suggestion in the issue, Form now adds a private symbol to the FormContext value it provides. When a Form sees that marker on the incoming context, it skips merging it (passing slot: null to useContextProps, the existing "don't use context" escape hatch). Fields are unaffected: they only read validationBehavior from FormContext, and still get it from their nearest Form.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices
• I understand every change in this PR and can explain why it's there.
• If AI-assisted, I followed our AI contribution guidance and pointed my assistant at AGENTS.md.

📝 Test Instructions:

yarn jest packages/react-aria-components/test/Form.test.js

• should not inherit props from a parent Form: renders a Form portaled inside another Form and checks the inner form has no inherited id/className, doesn't show the outer form's validation error, and submitting it doesn't call the outer onSubmit. Fails on main, passes with this change.
• should merge props from FormContext not provided by a Form: a user-provided FormContext.Provider between two forms still applies to the inner form.

Also ran the full RAC and S2 test suites and yarn check-types.

🧢 Your Project:

Personal contribution

[snowystinger]

Thanks for the quick fix!

[yihuiliao]

lgtm, can you sign the CLA? you'll need to close and re-open the PR to run the check

[snowystinger]

CLA instructions are here https://github.com/adobe/react-spectrum/blob/main/CONTRIBUTING.md#contributor-license-agreement

[UditDewan]

Thanks @yihuiliao and @snowystinger for the reviews and guidance! I signed the CLA and reopened the PR to rerun the checks.