ref(dynamic-sampling): Migrate projectSampling to new form system

[JonasBa]

Completes the form migration started in the previous PR by replacing the
custom formContext-based form in projectSampling with useScrapsForm.

What changed:

• projectSampling.tsx — replaces useFormState/FormProvider with useScrapsForm. A useEffect on the query data mirrors the old enableReInitialize: true behaviour, resetting the form (and savedProjectRates) whenever server data arrives or changes. Per-project rate validity is checked inline in the AppField render prop to disable the Apply Changes button, since z.record(z.string(), z.string()) is used for the schema (type safety only — Zod's ZodEffects chained on record values isn't assignable to FormValidateOrFn).
• projectsEditTable.tsx — removes the useFormField('projectRates') context consumer and instead receives projectRates, savedProjectRates, and onProjectRatesChange as explicit props. Per-project validation errors are now computed locally via getProjectRateErrors, keeping the validation logic co-located with the table that displays it.
• utils/projectSamplingForm.tsx — deleted.
• utils/formContext.tsx — deleted; this was the shared custom form context, now fully unused.

Stacks on top of #109356.

[getsantry]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀

[priscilawebdev]

I did manual testing, and everything looks good

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

[cursor]

Non-null assertions on potentially undefined project rates

Low Severity

savedProjectRates[project.id]! and projectRates[project.id]! use non-null assertions but can be undefined during the transition between data loading and the useEffect resetting the form. savedProjectRates is initialized via useState(projectRates) where projectRates is {} on first render, and is only populated after the useEffect fires — one render frame after isLoading becomes false. This causes inputs to receive undefined as their value prop, triggering React's uncontrolled-to-controlled warning (acknowledged and suppressed in the test). Using ?? '' instead of ! would provide a safe fallback and eliminate the transition.

Additional Locations (1)

• static/app/views/settings/dynamicSampling/projectSampling.tsx#L61-L63