Fix fetchOptions dispatcher with undici v8

[puneetdixit200]

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Changes being requested

Fixes #1886.

Node's built-in fetch can pass the older undici dispatch handler shape into a user-provided dispatcher. Newer undici@8 dispatchers reject that handler with UND_ERR_INVALID_ARG: invalid onRequestStart method, which breaks the documented fetchOptions.dispatcher proxy setup.

This wraps dispatcher calls with a compatibility adapter that forwards the older handler callbacks to the newer request lifecycle callbacks while leaving dispatchers that already receive the newer handler shape unchanged.

Additional context & links

Tests run:

• ./scripts/test tests/index.test.ts
• ./scripts/build
• built-package smoke test with undici@8.3.0 ProxyAgent verifying the failure changes from invalid onRequestStart method to a normal proxy connection error
• ./scripts/lint
• ./scripts/test
• git diff --check