mail-react: Enable TypeScript strict mode

[VPS-thodax]

Status quo

Strict mode is not enabled everywhere in this repo. @dextinity/mail-react extends tsconfig.core.json directly, which sets neither strictNullChecks nor noImplicitAny, so neither applied to this package so far.

Change

Enable strict for the package and fix the two resulting errors.

strictNullChecks also makes the return type of OneOfBlock and OptionalBlock nullable. Both already returned null in some branches, so this only corrects the emitted types — see the changeset.

Verification

Compared the emitted .d.ts files against main: those two return types are the only difference, so the changeset covers the full public type change. HtmlText.d.ts is byte-identical despite the removed index signature — it only ever served the implementation signature, which TypeScript does not emit.

Further information

• Task: https://vivid-planet.atlassian.net/browse/COM-1079

Note

Enable TypeScript strict mode in mail-react

• Adds "strict": true to tsconfig.json for @dextinity/mail-react
• Removes the string index signature from HtmlTextImplementationProps in HtmlText.tsx, narrowing props to declared fields only
• Makes OneOfBlock and OptionalBlock return types nullable to match existing runtime behavior, per the changeset
• Wraps a conditional render in a React.Fragment in MjmlWarningsPanel.tsx
• Risk: removing the [key: string]: unknown index signature on HtmlTextImplementationProps will cause type errors for any callers passing undeclared props

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[VPS-thodax]

Why dropping the index signature from HtmlTextImplementationProps is safe

HtmlTextImplementationProps types the implementation signature of HtmlText. TypeScript emits only the overload signatures of an overloaded function, never the implementation — so this type never reaches consumers. It is also not exported.

Checked rather than assumed:

• Built the package from main and from this branch and compared the emitted declarations. HtmlText.d.ts is byte-identical, and the type does not appear in it at all.
• Compiled a probe in the demo API against both builds: element="a" href, colSpan, element="div", plus an unknown attribute. Identical results in both, including that the unknown attribute is rejected either way — the overloads decide which props are accepted, the index signature never did.

Inside the implementation the remaining props are still spread onto the element exactly as before; only their static types are lost, and only within that function body.

The reason the index signature had to go: with strictFunctionTypes, an unknown-valued index signature makes the implementation signature incompatible with the element-specific overloads (TS2394). Dropping it was the smallest fix — the alternative was widening it to any plus an eslint-disable for no-explicit-any.

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