fix(runtime): return proxied responses at headers instead of body EOF

[Benjamin-eecs]

Summary

proxiedFetch used to await dispatcher.close(), and a graceful close waits for the active request. A streaming caller therefore did not receive the Response until body EOF.

What changed:

• Return the original native Response as soon as headers arrive.
• Start graceful dispatcher close without awaiting it. Undici keeps close pending until the active body reaches EOF, errors, or is cancelled.
• Compose caller cancellation and the time-to-headers timeout with AbortSignal.any, so abort after headers still reaches the original streamed body without a manual listener.
• Preserve the complete Fetch contract, including Response identity, URL metadata, redirect metadata, and immutable response headers.
• Timeout and request-error paths still destroy the dispatcher. timeoutMs: 0 remains supported.

No shared proxy pool and no global proxy setting changes.

Closes #2126

Verification

• @maka/runtime full suite: 3140 tests, 3131 passed, 0 failed, 9 skipped.
• Regression tests use a real local proxy and prove the Response arrives before the held body tail, the first chunk is readable immediately, EOF and cancel release resources, caller abort after headers terminates the original body, native response headers stay immutable, and both timeout paths remain bounded.
• npm run build:test passed.
• Biome and git diff --check passed.
• Desktop and e2e suites were not run; the change is confined to @maka/runtime.

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