cumulus: bump PARENT_SEARCH_DEPTH and add test for 12-core elastic scaling

[alindima]

On top of #6757

Fixes #6858 by bumping the PARENT_SEARCH_DEPTH constant to a larger value (30) and adds a zombienet-sdk test that exercises the 12-core scenario.

This is a node-side limit that restricts the number of allowed pending availability candidates when choosing the parent parablock during authoring.
This limit is rather redundant, as the parachain runtime already restricts the unincluded segment length to the configured value in the FixedVelocityConsensusHook (which ideally should be equal to this PARENT_SEARCH_DEPTH).

For 12 cores, a value of 24 should be enough, but I bumped it to 30 to have some extra buffer.

There are two other potential ways of fixing this:

• remove this constant altogether, as the parachain runtime already makes those guarantees. Chose not to do this, as it can't hurt to have an extra safeguard
• set this value to be equal to the uninlcuded segment size. This value however is not exposed to the node-side and would require a new runtime API, which seems overkill for a redundant check.

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/12464793256
Failed job name: build-rustdoc