urlapi: do not keep an internal port string

[icing]

Do not allocated an internal port string, just keep and number and a bit if it is present. Format the
port string in a local buffer when someone asks for it and copy that.

This avoid the format/alloc penalty when parsing URLs and no one ever asking for the port string.

(And there will be a method to get the uint16_t value directly in another PR)

[Copilot]

Pull request overview

This PR optimizes libcurl’s internal URL (CURLU) representation by removing the heap-allocated port string and instead storing only a numeric uint16_t port plus a “present” bit, formatting the port string only on demand. This reduces allocations and formatting work during URL parsing when callers never request the port string.

Changes:

• Remove the internal char *port field and associated allocation/free/duplication logic.
• Track explicit port presence via port_present and store the numeric port in uint16_t portnum.
• Format the port into a local buffer only when constructing a URL string or returning CURLUPART_PORT.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
lib/urlapi.c	Drops persistent port string storage; formats port lazily and updates same-origin/URL-get logic to use port_present.
lib/urlapi-int.h	Updates internal struct Curl_URL to replace char *port with uint16_t portnum + BIT(port_present).

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