types(dispatcher): use OutgoingHttpHeaders for request headers

[maruthang]

This relates to...

Closes #3840

Rationale

DispatchOptions.headers (and ConnectOptions.headers, UpgradeOptions.headers, ProxyAgent.Options.headers, Socks5ProxyAgent.Options.headers) are used to set headers on outgoing requests, but the type union UndiciHeaders was built around IncomingHttpHeaders (server-side semantics). This prevented users from passing common outgoing values like { 'content-length': 42 } (numeric values), even though the runtime accepts them and coerces them to strings via template-literal interpolation in lib/core/request.js.

This change introduces an OutgoingHttpHeaders alias in types/header.d.ts (mirroring Node core's http.OutgoingHttpHeaders: Record<string, number | string | string[] | undefined>), and uses it to model UndiciHeaders and the headers option on the proxy agents. Response/incoming-side fields (ResponseData.headers, ConnectData.headers, etc.) continue to use IncomingHttpHeaders as before.

Because IncomingHttpHeaders is a subtype of the new OutgoingHttpHeaders, this is a non-breaking widening of the request-header types: any code that previously typechecked still typechecks.

Changes

Features

N/A

Bug Fixes

• Add OutgoingHttpHeaders to types/header.d.ts and use it for outgoing-request headers options:
  • Dispatcher.UndiciHeaders (DispatchOptions, ConnectOptions, UpgradeOptions)
  • ProxyAgent.Options.headers
  • Socks5ProxyAgent.Options.headers
• Update docs/docs/api/Dispatcher.md UndiciHeaders parameter docs to reflect OutgoingHttpHeaders semantics (matches the screenshot in Typing issue: headers in DispatchOptions should use OutgoingHttpHeaders #3840).
• Add tsd regression tests in test/types/header.test-d.ts and test/types/dispatcher.test-d.ts covering numeric header values and Node core OutgoingHttpHeaders interop.

Breaking Changes and Deprecations

None. OutgoingHttpHeaders is a strict superset of IncomingHttpHeaders for the value union, so existing usages continue to typecheck.

Status

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