Configurable pgpool/pgDog backends (multi-emit)

[raivil]

What

Generalizes the single primary-pinning comment into a configurable, multi-valued PgpoolNoLoadBalance.backends. The gem can now emit the pgpool hint, the pgDog hint, or both at once during a migration.

Backend	Comment emitted
:pgpool (default)	/*NO LOAD BALANCE*/
:pgdog	/* pgdog_role: primary */

PgpoolNoLoadBalance.backends = [:pgpool]          # default — unchanged behavior
PgpoolNoLoadBalance.backends = [:pgpool, :pgdog]  # /*NO LOAD BALANCE*/ /* pgdog_role: primary */ SELECT ...
PgpoolNoLoadBalance.backends = "pgpool,pgdog"     # comma-string also accepted

Emission order is always pgpool-first regardless of input order. Unknown/empty backends raise ArgumentError. Default [:pgpool] keeps existing behavior identical.

Why both comments work behind either router (verified against source)

• Pgpool-II: allow_sql_comments defaults to off; is_select_query() routes any SELECT not starting with s/(/w/t to the primary, so any leading comment qualifies.
• PgDog: parse_edge_comment merges all consecutive leading block comments into one slice before regex-scanning for pgdog_role: (proven by its own test_remove_comment_multiple_leading), so the directive in the second comment is still found.

Compatibility

• Renames the relation/model/arel API to no_load_balance; pgpool_nlb remains a full alias (method, execute: keyword, unscope symbol).
• Preserves PgpoolNoLoadBalance.force and the Rails 8 4-arg to_sql_and_binds (the abandoned feature/pgdog-backend-support branch dropped both — this branch is built on cm instead).
• ExplainSubscriber strips every known backend comment.

Tests

bundle exec rspec → 39 examples, 0 failures.

Deployment prerequisites (not code-enforced)

• Pgpool: allow_sql_comments off (default).
• PgDog: comment-based manual routing / parser enabled.