Fix exit code for postgres config patch / reset-password usage errors

[sdairs]

What

cloud postgres config patch with neither --set nor --file, and cloud postgres reset-password with neither --password nor --generate, exited 1 instead of the documented usage-error exit code 2.

Why

The "provide at least one of X or Y" checks lived in the handler as a runtime CloudError, so they went through the generic error path (exit 1) instead of clap's usage-error path (exit 2). config replace already gets this right because --file is a plain required field there.

Fix

Added ArgGroups (required(true)) over the existing conflicts_with pairs so clap itself enforces "at least one of":

• PostgresCommands::ResetPassword: group over password / generate.
• ConfigCommands::Patch: group over sets / file.

The handler-side checks are now unreachable through the CLI and were converted to debug_assert!/unreachable! safety nets, matching the existing pattern already used for the conflicts_with combination in postgres_reset_password.

Test plan

• Added clap parse tests: rejects_postgres_config_patch_without_set_or_file and rejects_postgres_reset_password_without_password_or_generate, both asserting clap::error::ErrorKind::MissingRequiredArgument (which maps to exit code 2) and that the error message names both valid flags.
• Confirmed existing valid-combination tests (--set, --file, --password, --generate, and the mutual-exclusion rejections) still pass.
• cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test -p clickhousectl all clean.

Fixes #613

Note: this PR is part of a stacked chain — its base is fix/610-queries-file-multi-statement, not main.

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