refactor: improve multi_socket_cb implementation

[imor]

This PR improves the multi_socket_cb implementation for both kqueue and epoll by doing the following:

For kqueue:

• Fixes the leaks in the filters because the sock_info->action was not being updated. This also lead to a use-after-free bug and sometimes made the tests on macOS fail due to a segfault. All these bugs are fixed.
• Correctly checks the return value of curl_multi_assign and raises a Postgres error if the call fails.

For epoll:

• Correctly checks the return value of curl_multi_assign and raises a Postgres error if the call fails.
• Correctly handle the case when what == CURL_POLL_INOUT by setting the right EPOLLIN and EPOLLOUT combination in ev.events member.
• Sets a sentinel value in the call to curl_multi_assign the first time multi_socket_cb is called for a socket fd and removes it when what == CURL_POLL_REMOVE. This clarifies the intent of the curl_multi_assign call because the older code assignments to socket_exists variable were never read and were confusing.

[steve-chavez]

@imor Can we prefix this PR as refactor: instead of fix:? Two reasons:

• It doesn't need a release since Macos is not used for production. Otherwise it could be confusing for packagers like supabase/postgres
• Macos compatibility was only provided for development purposes.

[imor]

@steve-chavez yeah I intend to clean this PR up. I added this as a draft so that I don't forget. I only needed this change to run the tests on my local setup.

[steve-chavez]

@imor Looks good! On second thought I'll change this to a fix: commit since it also changes the logic for epoll. That way the change clearly shows on the release (in case if something goes wrong).

[steve-chavez]

A further improvement could be to run the loadtest on macos, that should prove pg_net works the same there

pg_net/.github/workflows/main.yml

Lines 56 to 78 in 0ab4bae

	loadtest:
	runs-on: ubuntu-latest
	strategy:
	matrix:
	params: [ {reqs: 10000, batch: 200}, {reqs: 20000, batch: 400}, {reqs: 40000, batch: 800} ]
	steps:
	- uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
	- name: Install Nix
	uses: cachix/install-nix-action@c134e4c9e34bac6cab09cf239815f9339aaaf84e # v31.5.1
	- name: Use Cachix Cache
	uses: cachix/cachix-action@3ba601ff5bbb07c7220846facfa2cd81eeee15a1 # v16
	with:
	name: nxpg
	authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
	- name: Build
	run: nix-shell --run "xpg build"
	- name: Run load test
	run: |
	nix-shell --run "net-loadtest ${{ matrix.params.reqs }} ${{ matrix.params.batch }}" >> "$GITHUB_STEP_SUMMARY"