Split --label on the first '=' only so values may contain '='

[chengboonrong]

Summary

container run --label KEY=VALUE rejected any label whose value contained a
=:

$ container run --rm --label 'config=key=value' alpine true
Error: invalid label format config=key=value

This blocks common real-world values such as reverse-proxy router rules
(--label 'traefik.http.routers.x.rule=Host(ex.com)') and nested
key=value configuration.

Root cause

Parser.labels split each entry with split(separator: "=", maxSplits: 2) and
only handled 1 or 2 parts, throwing for anything else. A value containing a =
produces 3+ parts and hits the default (throw) branch.

Fix

Split on the first = only (maxSplits: 1), keeping everything after it as
the value. This matches docker run --label and the env-file parser already in
this same file (URL=https://foo.bar?baz=woo is accepted). Entries with no =
still map to an empty value; behavior for all previously-valid labels is
unchanged.

Tests

Adds a ParserTest case covering a value with =, a Traefik-style rule, a
plain key=value, a bare key, and an empty value. Verified locally:
swift build, swift format lint --strict, and the parser tests all pass.

Fixes #1977

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