[flags] Cleanup enableHalt

[everettbu]

Mirror of facebook/react#35708
Original author: eps1lon

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This has shipped in 19.2

[greptile-apps]

Greptile Overview

Greptile Summary

• Removes the enableHalt feature flag and all associated conditionals in React Fizz/Flight server implementations so halt semantics are unconditional for prerender abort paths.
• Simplifies static prerender APIs (Browser/Edge/Node) to always return StaticResult with a postponed field.
• Updates and ungates a large set of DOM/Flight prerender tests to reflect the now-always-on halt behavior, including pruning the previous “halt disabled” expectations.
• Adjusts a few stack-location inline snapshots in Flight DOM browser/node tests after line shifts.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk.
• Changes are a straightforward removal of a now-always-on feature flag plus corresponding test expectation updates; the head commit also confirms there are no remaining enableHalt references under packages, and the diff introduces no whitespace issues.
• No files require special attention

Important Files Changed

Filename	Overview
packages/react-dom/src/tests/ReactDOMFizzStaticBrowser-test.js	Removes enableHalt conditional branches and disable-mode tests; updates gates on SuspenseList/OptimisticKey tests accordingly.
packages/react-dom/src/tests/ReactDOMFizzStaticNode-test.js	Drops enableHalt gating and removes disable-mode tests/branches; assumes prerender abort returns empty prelude with errors.
packages/react-dom/src/server/ReactDOMFizzStaticBrowser.js	Removes enableHalt flag usage; always includes postponed in StaticResult from prerender.
packages/react-server-dom-webpack/src/tests/ReactFlightDOMBrowser-test.js	Removes enableHalt gating for prerender tests and adjusts stack-location inline snapshot numbers after earlier edits.
packages/react-server-dom-webpack/src/tests/ReactFlightDOMNode-test.js	Removes enableHalt gating comments and updates stack-location inline snapshots to new line numbers.
packages/react-server/src/ReactFizzServer.js	Eliminates enableHalt conditionals; prerender abort paths now always use tracked-postpones/segment checks to halt tasks.
packages/react-server/src/ReactFlightServer.js	Eliminates enableHalt flag checks; PRERENDER abort behavior always halts tasks for thenables/streams/blobs and abort().
packages/shared/ReactFeatureFlags.js	Removes the enableHalt feature flag export.
packages/shared/forks/ReactFeatureFlags.native-fb.js	Removes enableHalt export from native-fb feature flags fork.
packages/shared/forks/ReactFeatureFlags.test-renderer.js	Removes enableHalt export from test-renderer feature flags fork.
packages/shared/forks/ReactFeatureFlags.www.js	Removes enableHalt export from www feature flags fork.

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