Feat: Add optional execution observations

[mday-io]

Summary

Add optional start/finish callbacks so integrations can record execution failures and associate SQL logs without patching SQLMesh internals.

• Cover plan stages, model batches, physical and virtual updates, audits and base-adapter queries.
• Isolate ordinary callback errors; consumers own IDs, timing and storage.
• Query callbacks finish before transaction completion and result fetching; some adapter paths bypass them.
• Materialization reports the logical target during write-audit-publish (WAP); finish precedes later audits and publication.
• Depends on Fix: Propagate terminal worker failures and stop queued DAG work #6140 and Feat: Add optional per-task context propagation #6141. The current diff includes both prerequisites.

Test Plan

• Tested callback identity, recursion, errors, disabled behavior and registration.
• Tested serial/parallel dispatch, cancellation, transaction-exit failures and WAP targets.
• Verified stacked production code and docs match the original PR.
• Ran make style and all make fast-test batches: 2,914 passed.
• Run affected downstream consumer tests against the current stack.
• Confirm current-head CI and diagnose Windows failures.