feat(react-email): warn when content is dropped silently

Closes #460

Two cases in renderToHtml / renderToPlainText dropped content with no signal, which is hard to notice with LLM-generated specs: the email renders, just missing pieces.

Changes

• Unknown component type: console.warn naming the type and element key before returning null.
• Children on a standard leaf component (slots: [] in standardComponentDefinitions, e.g. Link > Image): console.warn with the count of dropped children. It only fires when the registry entry is the standard component, so custom overrides that do render children stay silent.

Warnings only, no behavior change. Same idea as the slot check in the Chat SDK renderer (#451), which rejects children under components without slots.

Tests

• 4 new tests: both warnings, no warning for a custom override that renders children, no warning for a valid example spec. The two warning tests fail without the change.
• packages/react-email: 37/37 pass, tsc --noEmit clean.

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