Fix setting of global behavior flags

[MichelleArk]

resolves #351

Problem

As part of #323, we had introduced the require_batched_execution_for_custom_microbatch_strategy behaviour flag as part of the BaseAdapter._behavior_flags method. However, any concrete adapters that overwrote this property effectively removed the new behaviour flag from the collection, which would lead to downstream CompilationErrors (behaviour flag not registered).

Solution

Set up require_batched_execution_for_custom_microbatch_strategy as a global behaviour flag.

Also added a test to ensure BaseAdapter._behaviour_flags is always empty to avoid this issue in the future.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX

[MichelleArk]

✅ Testing against dbt-snowflake: https://github.com/dbt-labs/dbt-snowflake/actions/runs/11806247075

This is the only first-party adapter that overrides BaseAdapter._behaviour_flags and would have encountered this issue.

[MichelleArk]

✅ dbt-postgres testing: https://github.com/dbt-labs/dbt-postgres/actions/runs/11806285678

Mostly a sanity test for broader functional coverage.