Split revalidation errors by execution context (#98993)
## Summary
- Narrow build-time generator names to a literal type that can be
extended as new generators are added.
- Split the combined revalidation diagnostic into render,
cached-function, and build-time-generator errors so each failure reports
only its actual context and never needs a fallback generator name.
- Cover all three contexts in the existing development and production
test suites.
Follow-up to the review feedback on #98891.
## Verification
- `HEADLESS=true pnpm test-dev-webpack
test/e2e/app-dir/revalidatetag-rsc/revalidatetag-rsc.test.ts` (3 tests)
- `HEADLESS=true pnpm test-start-webpack
test/e2e/app-dir/revalidatetag-rsc/revalidatetag-rsc.test.ts` (3 tests)
- Production Webpack `generate-static-params-errors` suite (6 tests)
- `pnpm --filter=next types`
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