feat: optionally preserve sequences in WandB logs

[nightcityblade]

Fixes #3809

Description:

Add an opt-in flatten_sequences=False setting to the Weights & Biases output handler so lists of scalar metric values can be logged as one sequence. Existing callers retain the current flattened-per-index behavior by default.

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[nightcityblade]

Rebased the branch onto current master and adapted the change to the latest BaseOutputHandler refactor, resolving the merge conflict. Verification: Ruff check and format check pass on all three changed files; the full WandB handler test module passes (13 tests).