fix: replace assert statements with raise ValueError in EncDecMultiTaskModel

[stanley1208]

What does this PR do?

Replace 5 assert statements with explicit raise ValueError in EncDecMultiTaskModel and its prompt parsing utility, per the Contributor guidelines which state:

"RaiseError is preferred to assert"

assert statements are silently removed when Python runs with the -O (optimize) flag, meaning invalid inputs would pass through uncaught in production. raise ValueError always executes regardless of optimization flags.

Changes:

• Line 549: Validate timestamps parameter with raise ValueError instead of assert
• Line 614: Validate use_lhotse config with raise ValueError instead of assert
• Line 1415: Validate multi-turn prompt format with raise ValueError instead of assert
• Line 1427: Validate prompt dict values with raise ValueError instead of assert
• Line 1441: Validate prompt["slots"] type with raise ValueError instead of assert

Collection: ASR

Changelog

• Replace 5 assert statements with raise ValueError in aed_multitask_models.py
• All error messages preserved from original assertions

Before your PR is "Ready for review"

• Make sure you read and followed Contributor guidelines
• Did you write any new necessary tests? (N/A — no behavior change, error type changes from AssertionError to ValueError)
• Did you add or update any necessary documentation?

PR Type:

• Bugfix

[stanley1208]

@nithinraok Ready for review — replaces 5 assert statements with raise ValueError per CONTRIBUTING.md guidelines. No behavior change, just safer error handling. Thanks!

[pzelasko]

Looks good, please fix the checks