Third of the `http_proxy` stack. Stacked on #59217 and the
upstream-config PR — review/merge those first.

Adds the in-process HTTP/HTTPS proxy server that enforces an
`Allowlist`. It speaks HTTP CONNECT for HTTPS tunnels and forward
proxying for plain HTTP, vets resolved addresses against
loopback/private/link-local ranges to prevent DNS-rebinding past the
sandbox, pins each connection to the destination approved for its first
request (so later keep-alive requests can't escape the policy decision),
optionally chains through the upstream proxy, and bounds header sizes,
connection counts, and connect/handshake waits since its sole client is
untrusted model-driven code running inside the editor process. Includes
end-to-end tests covering allowed/denied CONNECT and HTTP forward,
IP-literal handling, DNS-rebinding denial, and upstream chaining.

Still has no callers; wired into the agent terminal sandbox in the
remaining PRs of the stack.

Release Notes:

- N/A

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Co-authored-by: MartinYe1234 <52641447+MartinYe1234@users.noreply.github.com>