[Backport 2026.1] size based load balancing: add test

[scylladbbot]

Test to ensure that the cluster is balanced correctly in difficult conditions:

• heterogeneous nodes with different disk sizes.
• multiple tables with different partition sizes.

refs: scylladb/scylladb#24724

This test will:

1. Expand the cluster by adding nodes of different instance types.
2. Write the first half of data
3. Add some nodes to the cluster
4. Write the second half of data
5. Remove the added nodes
6. Wait for tablet migrations to finish
7. Check the balance of the cluster

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Cluster setup: 1 DC, 3 racks, 3 nodes per rack (1x i4i.large, 2x i4i.2xlarge)
At steps 3 and 5 the nodes are i4i.large

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Each half of data is in 3 distinct tables, with this configuration:
The test creates 3 tables:

• 1st table

  • 200000 x 1MB partitions
  • 10000 x 10MB partitions
  • 200 x 1GB partitions
  • 10 x 10GB partitions

• 2nd table

  • 100000 x 1MB partitions
  • 2000 x 100MB partitions
  • 100 x 1GB partitions
  • 2 x 100GB partitions

• 3rd table

  • 20000 x 10MB partitions
  • 1000 x 100MB partitions
  • 20 x 10GB partitions
  • 1 x 100GB partitions

  ---

Example results

$${\color{green}green}$$ = nodes of the rack are balanced within 5%
$${\color{yellow}yellow}$$ = nodes of the rack are not balanced (more than 10%)

Testing

• before 2025.4.2-20260116.64039588dbe6
  

• after 2026.2.0~dev-20260125.55422593a7f7

PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)

• Add unit tests to cover my changes (under unit-test/ folder)

• Update the Readme/doc folder relevant to this change (if needed)

• (cherry picked from commit cf3f876)

• (cherry picked from commit cbc599a)

• (cherry picked from commit 065fdee)

Parent PR: #10992