Allow selecting database diagnostics checks

What

Allow running database diagnostics checks individually instead of only all at once.

Why

Reviewer suggestion on !251254 (merged): !251254 (comment 3729162360)

Changes

• Gitlab::Database::Diagnostics::Console::VIEWS is now a hash keyed by check name ('search_path' => Views::SchemaResolution, 'autovacuum_settings' => Views::AutovacuumSettings) instead of an array.
• Console.run accepts check_names: and selects views from the registry. Nil or empty runs all checks, repeated names run once, names may be strings or symbols, and an unknown name raises Console::UnknownCheckError with the valid list.
• Per-check rake tasks are generated from a slug list in the rake file, e.g. gitlab:db:diagnostics:search_path[database_names] and gitlab:db:diagnostics:autovacuum_settings[database_names]. The slug list is duplicated in the rake file because Zeitwerk autoloading is unavailable at rake load time. A spec fails if the rake list and the registry drift.
• Shared task body moved to Gitlab::Database::Diagnostics::RakeTask, since RuboCop (Rake/TopLevelMethodDefinition) forbids top-level methods in rake files.
• Console::Runner requires views:. Its old default of every view was unreachable, since Console.run always passes the selection.

Behavior is unchanged when no checks are requested: same output, same 0/1 exit code contract.

Usage

# all checks
bundle exec rake gitlab:db:diagnostics

# one check
bundle exec rake "gitlab:db:diagnostics:search_path[main]"

# autovacuum settings check
bundle exec rake "gitlab:db:diagnostics:autovacuum_settings[main]"

How to verify

bundle exec rspec spec/lib/gitlab/database/diagnostics spec/tasks/gitlab/db/diagnostics_rake_spec.rb

97 examples, all passing. Also verified with a live run of the per-check rake task.