Fix false hydration mismatch when client renders portal that returned null on server

[starboyvarun]

Summary

Fixes #12615

When a component returns null on the server but renders a ReactDOM.createPortal() on the client, React was incorrectly emitting a hydration mismatch error:

Hydration failed because the server rendered HTML didn't match the client.
  <span>
    <HoverMenu>
+     <div>   ← false positive: this is portal content, not a span child

This is a false positive — the portal's <div> renders into a separate container, not into the parent <span>. It should not be compared against the parent's server HTML.

Root Cause

When updatePortalComponent calls pushHostContainer to switch to the portal's container, the hydration state variables (isHydrating, nextHydratableInstance, hydrationParentFiber) were not reset. The portal's children then tried to claim hydration instances from the parent container's server nodes, which failed and triggered a mismatch error.

Fix

Added two functions to ReactFiberHydrationContext:

• prepareToHydrateHostPortal — called in updatePortalComponent (begin work). Saves the current hydration state onto a stack and resets it, so portal children are inserted fresh rather than matched against server HTML.
• popHydrationStateAfterPortal — called in the HostPortal case of complete work. Restores the saved hydration state so the parent resumes hydrating correctly after the portal.

This correctly handles nested portals (via the stack) and cleans up on resetHydrationState (work loop interruption).

Test Plan

• Added two new tests in ReactDOMHydrationDiff-test.js:
  • Portal renders to a separate container → no mismatch warning
  • Portal renders to the hydration root container → no mismatch warning
• All 39 hydration diff tests pass
• All 183 tests across ReactServerRenderingHydration, ReactDOMServerPartialHydration, and ReactDOMFiber suites pass

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