fix(tests): tolerate new cardano-cli 'Missing:' spacing

[mkoura]

The cardano-cli optparse error output now pads 'Missing:' with variable whitespace (e.g. 'Missing: --socket-path SOCKET_PATH') and can render empty alternation groups inline
('Missing: ( ) ( ) --fee LOVELACE').

Replace exact substring assertions with regex matching in test_env_network_id, test_socket_path and test_missing_fee so both old and new formats match.

[Copilot]

Pull request overview

This PR updates several negative-path E2E tests to be resilient to recent cardano-cli optparse formatting changes, specifically variable whitespace after Missing: and altered rendering of required-argument listings, by switching from exact substring checks to regex-based matching.

Changes:

• Relax --fee missing-argument assertion to accept both legacy and updated cardano-cli outputs.
• Switch socket-path and network-id missing-argument checks from substring matching to regex matching (including escaping where appropriate).
• Update skip logic in test_env_network_id to detect the missing-network-id condition via regex.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
cardano_node_tests/tests/test_tx_negative.py	Replaces a fragile exact missing-fee substring assertion with a regex-backed check while retaining legacy error variants.
cardano_node_tests/tests/test_socket_path.py	Adds regex matching for optparse Missing: spacing changes and escapes literal patterns to keep behavior equivalent for non-optparse errors.
cardano_node_tests/tests/test_env_network_id.py	Converts the missing network-id error detection from substring to regex (including in the skip-on-missing-env fixture).

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