Publish: Warn when keyring has no password

[konstin]

When trying to upload without a password but with the keyring, check that the keyring has a password for the upload URL and username and warn if it doesn't.

Fixes #8781

[zanieb]

Did you know we have a dedicated test keyring plugin? Might make life easier

uv/crates/uv/tests/it/lock.rs

Lines 16221 to 16272 in cc6bfa1

	// Install our keyring plugin
	keyring_context
	.pip_install()
	.arg(
	keyring_context
	.workspace_root
	.join("scripts")
	.join("packages")
	.join("keyring_test_plugin"),
	)
	.assert()
	.success();
	let context = TestContext::new("3.12");
	let pyproject_toml = context.temp_dir.child("pyproject.toml");
	pyproject_toml.write_str(
	r#"
	[project]
	name = "foo"
	version = "0.1.0"
	requires-python = ">=3.12"
	dependencies = ["iniconfig"]
	[build-system]
	requires = ["setuptools>=42"]
	build-backend = "setuptools.build_meta"
	[tool.uv]
	keyring-provider = "subprocess"
	[[tool.uv.index]]
	name = "proxy"
	url = "https://pypi-proxy.fly.dev/basic-auth/simple"
	default = true
	"#,
	)?;
	// Provide credentials via environment variables.
	uv_snapshot!(context.filters(), context.lock()
	.env(EnvVars::index_username("PROXY"), "public")
	.env(EnvVars::KEYRING_TEST_CREDENTIALS, r#"{"pypi-proxy.fly.dev": {"public": "heron"}}"#)
	.env(EnvVars::PATH, venv_bin_path(&keyring_context.venv)), @r###"
	success: true
	exit_code: 0
	----- stdout -----
	----- stderr -----
	Request for public@https://pypi-proxy.fly.dev/basic-auth/simple/iniconfig/
	Request for public@pypi-proxy.fly.dev
	Resolved 2 packages in [TIME]
	"###);

[konstin]

That's neat!