fix: support frozen globalThis

[ronag]

Summary

• adapt fix: handle frozen globalThis in setGlobalDispatcher nodejs/undici#5574 so importing and configuring the lower fork works when globalThis is frozen
• preserve setGlobalDispatcher() / getGlobalDispatcher() replacement semantics when the freeze happens either before import or after the initial dispatcher is published
• keep the custom live-socket registry usable when a frozen global cannot accept its symbol and legacy-alias properties
• cover both freeze timings in isolated child processes, including dispatcher identity and socket registration/removal

Upstream fix: nodejs#5574

Validation

Exact head: 286f1dabc3a5020fdcc276e3381e449731ebb66f on Node.js v26.7.0

• frozen-global regression: 2 passed
• socket-registry regression: 4 passed
• Agent/global-dispatcher regression: 25 passed
• full npm test: unit 576 passed, 6 skipped; node-test 151 passed; type tests passed
• npm run lint
• git diff --check

[Copilot]

Pull request overview

This PR hardens Undici’s global dispatcher and live-socket registry publishing logic to work correctly when globalThis is frozen, and adds regression tests to cover freezing both before and after import.

Changes:

• Add a module-local fallback dispatcher in lib/global.js so setGlobalDispatcher() / getGlobalDispatcher() continue to work when globalThis cannot be mutated.
• Make the socket registry resilient to non-extensible/frozen globals by avoiding ??= publication to a Symbol.for slot when the global cannot accept new properties, while keeping the legacy alias behavior when possible.
• Add isolated child-process tests covering both “freeze before import” and “freeze after import” scenarios, including dispatcher identity and socket tracking/removal.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
test/global-frozen.js	Runs a fixture in two freeze phases to validate frozen-global behavior in isolated processes.
test/fixtures/frozen-global.js	Exercises dispatcher replacement semantics and socket registry behavior under a frozen globalThis.
lib/global.js	Introduces a fallback dispatcher path when defining the global symbol property fails due to a frozen/non-extensible global.
lib/core/socket-registry.js	Avoids publishing new global symbol/alias properties when globalThis is non-extensible, while preserving cross-copy sharing when already published.

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