docs(custom-validation): fix misleading setInvalid usage and clarify limitations [DX-1077]

[Lisa White (whitelisab)]

Purpose

The custom-validation example has been calling sdk.entry.fields['slug'].getForLocale(...).setInvalid() from within a different field's location — a pattern that silently does nothing. This has caused confusion for multiple developers (#3992), who expected the call to show a visual error indicator on the slug field.

Two related limitations were not documented anywhere in the example:

• setInvalid is visual only — it adds/removes the red error bar but does not prevent publishing
• setInvalid only affects the field the app is assigned to — cross-field calls are no-ops

Approach

• Field.tsx: Removed the two cross-field setInvalid calls. The publish-blocking logic (sdk.field.setValue('true'/'false')) was already correct and is unchanged — this just removes the dead code that was misleading readers.
• README.md: Updated the description of setInvalid to correctly describe it as a visual-only, same-field-only indicator, and explicitly points readers to the setValue pattern for preventing publishing.

No functional behavior changes — this is documentation and example cleanup only.

Testing steps

1. Bootstrap the example: npx create-contentful-app --example custom-validation
2. Set up the content type with a validation short-text field (must equal "true") per the README
3. Assign the app to the validation field via the Appearance tab
4. Enter an invalid slug format — confirm the error message renders in the app UI
5. Confirm the entry cannot be published while the slug is invalid
6. Fix the slug format — confirm the entry publishes successfully
7. Confirm no red error bar appears on the slug field (the old misleading behavior is gone)

Breaking Changes

None.

Dependencies and/or References

• Closes #3992
• DX-1077
• Related: companion PRs in doc-app and ui-extensions-sdk will add the same clarifications to the developer docs and SDK type definitions

Deployment

N/A — example and documentation changes only.