docs(clickstack): clarify dashboard validation and update bodies

[sdairs]

Problem

The dashboard documentation said dashboard validate could check the same body before either create or update. The validation command accepts the create dashboard schema, while updates use a distinct full replacement schema whose existing filters require their id values.

Result

• Describe dashboard validate as create-body validation only.
• Keep the dashboard update full replacement warning and tell users to preserve every existing filter ID.
• Add a concrete update filter example and align the validate/update command help with those schemas.

Schema evidence

The typed validation method accepts ClickStackCreateDashboardRequest, whose filters are ClickStackFilterInput values without an id. ClickStackUpdateDashboardRequest instead uses ClickStackFilter, where id is required.

Validation

• cargo fmt --all
• cargo test -p clickhousectl clickstack::tests
• cargo clippy -p clickhousectl -- -D warnings
• cargo check -p clickhousectl --no-default-features
• cargo clippy -p clickhousectl --all-targets --no-default-features -- -D warnings
• Rendered dashboard validate --help and dashboard update --help

Refs #693

Follow-up to #718

[iskakaushik]

LGTM