Classify scheduled function parameter errors

[UllasSHR]

Description of Changes

Scheduled function parameter preparation previously returned generic errors, so both scheduler paths logged datastore and BSATN failures, stale schedules, and invalid saved arguments at the same error level.

This change introduces typed errors so the scheduler can preserve that distinction:

• datastore and BSATN encoding failures use error!
• missing reducers or procedures and invalid saved arguments use warn!

Both scheduled procedure and reducer paths use the classification. A unit test covers all six error categories.

Addresses #5767.

API and ABI breaking changes

None.

Rollback safety impact

n/a

Expected complexity level and risk

2 - The change is limited to scheduler error classification and logging. It does not change public APIs, database storage, or scheduling behavior.

Testing

• cargo test --locked -p spacetimedb-core --lib
• cargo fmt --check -p spacetimedb-core
• git diff --check