Remove disableClientCache feature flag

[ShehabSherif0]

Summary

The disableClientCache feature flag is set to true in all environments (main flags file, www, native-fb, native-oss, test-renderer, test-renderer.www, test-renderer.native-fb). This PR removes the flag and the dead code path it guarded.

When disableClientCache was true, ReactCacheClient.js exported noopCache and noopCacheSignal (no-ops that pass through or return null). With the flag hardcoded to true everywhere, the conditional branch that imported the real cache implementation from ReactCacheImpl.js was unreachable dead code.

This PR:

• Removes the disableClientCache flag from ReactFeatureFlags.js and all 6 fork files
• Simplifies ReactCacheClient.js to unconditionally export the no-op implementations
• Removes the __VARIANT__ test override in setupTests.www.js
• Removes the @gate disableClientCache annotation from a test in ReactMarkupClient-test.js

Note: ReactCacheServer.js is unaffected -- it imports directly from ReactCacheImpl.js without any flag check.

How did you test this change?

• yarn linc (lint on changed files): passed
• yarn test (dev mode, experimental): all cache and markup tests passed (23/23)
• yarn test --prod (production mode, experimental): all tests passed (23/23)
• yarn test -r www-modern (www variant): all tests passed (6/6)
• yarn flow (Flow type checks): 0 errors
• Verified zero remaining references to disableClientCache across the codebase

[Copilot]

Pull request overview

Removes the disableClientCache feature flag (previously hardcoded true everywhere) and simplifies client-side React.cache to always use the no-op implementation, eliminating unreachable flag-guarded code paths.

Changes:

• Deleted disableClientCache from ReactFeatureFlags.js and all relevant fork flag files.
• Simplified packages/react/src/ReactCacheClient.js to unconditionally export the no-op cache/cacheSignal.
• Removed the www Jest test override and the @gate disableClientCache annotation from a markup client test.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
scripts/jest/setupTests.www.js	Removes the __VARIANT__ override for a now-deleted flag in www Jest setup.
packages/shared/ReactFeatureFlags.js	Removes the disableClientCache flag definition from shared feature flags.
packages/shared/forks/ReactFeatureFlags.www.js	Removes the flag from the www fork.
packages/shared/forks/ReactFeatureFlags.test-renderer.www.js	Removes the flag from the test-renderer www fork.
packages/shared/forks/ReactFeatureFlags.test-renderer.native-fb.js	Removes the flag from the test-renderer native-fb fork.
packages/shared/forks/ReactFeatureFlags.test-renderer.js	Removes the flag from the test-renderer fork.
packages/shared/forks/ReactFeatureFlags.native-oss.js	Removes the flag from the native-oss fork.
packages/shared/forks/ReactFeatureFlags.native-fb.js	Removes the flag from the native-fb fork.
packages/react/src/ReactCacheClient.js	Makes cache and cacheSignal always export no-ops (no conditional import/branch).
packages/react-markup/src/tests/ReactMarkupClient-test.js	Removes @gate disableClientCache since the flag no longer exists.

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[MorikawaSouma]

Nice cleanup! Removing dead code paths simplifies the codebase. The changes look correct - the flag was \ rue\ everywhere, so the conditional branch was never executed.