fix: reject repositories.<name>.url in config

[lawrence3699]

Fixes #10570.

Before

poetry config repositories.repo2.url https://... is accepted even though the documented CLI syntax is poetry config repositories.<name> <url>.

That writes a nested [repositories.repo2.url] table to config.toml, which then makes commands such as poetry config --list fail with unhashable type: 'dict'.

After

Reject repositories.<name>.url when setting repository URLs and point users to the documented repositories.<name> form instead.

This keeps the existing repositories.<name>.url read path intact while preventing the bad write that corrupts the config structure.

Validation

• .venv/bin/pytest tests/console/commands/test_config.py -q
• manual repro with POETRY_CONFIG_DIR=$(mktemp -d)
  • before: poetry config repositories.repo2.url https://... corrupted config.toml and poetry config --list failed
  • after: the invalid write is rejected and poetry config --list succeeds