Correct local Postgres diagnostics and help

[sdairs]

Summary

• Route local Postgres validation and runtime failures through dedicated errors so they never report a ClickHouse execution failure.
• Honor DOCKER_HOST endpoint selection and give constructor and ping failures platform-aware Docker Desktop or Engine, socket permission, context, and rootless guidance.
• Correct start help for the versioned data path, all five managed labels, and omitted versus explicit port behavior.
• Add exact help snapshots plus subprocess fake-Docker coverage for missing sockets, permission failures, daemon-down responses, and missing psql.

Tests

• cargo test -p clickhousectl (768 passed)
• cargo test -p clickhousectl --test local_postgres_diagnostics_test (4 passed)
• cargo test -p clickhousectl --test local_postgres_help_test (2 passed)
• cargo test -p clickhousectl --test local_postgres_preflight_test (2 passed)
• cargo fmt --all --check
• cargo clippy -p clickhousectl --all-targets -- -D warnings
• cargo build -p clickhousectl

Stack

Position 4 of 4 (top), based on #501.

• Validate local Postgres start options before Docker work #486 validates local Postgres start options before Docker work.
• Wait for PostgreSQL readiness before local start success #498 waits for PostgreSQL readiness before reporting startup success.
• Make local Postgres startup transactional #501 makes fresh startup transactional across Docker start, metadata persistence, and readiness.
• This PR corrects Postgres and Docker diagnostics and pins the rendered help contract.

Closes #465

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