Add default parameter to task and asset state get() accessors

[amoghrajesh]

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Was generative AI tooling used to co-author this PR?

• Yes: claude sonnet 4.6

closes: #67841

What?

TaskStoreAccessor.get() and AssetStoreAccessor.get() returned None when a key did not exist, which made callers to use None checks or or based fallbacks like state.get("watermark") or "2026-01-01". This is not the best authoring experience and does not follow the standard Python dict.get(key, default) pattern.

Current behaviour

state.get("watermark") returns None if the key has never been set. Callers must write state.get("watermark") or "fallback", which is both verbose for no benefit.

Proposed change

• Added a default parameter to get() on TaskStoreAccessor and AssetStoreAccessor, and the AssetStoreAccessors shorthand accessor. The default is None. Example usage:

state = context["task_store"]
watermark = state.get("watermark", default="2026-01-01T00:00:00+00:00")
total_runs = state.get("total_runs", default=0)

• set(key, None) now raises ValueError — None is not a valid state value

The example dag example_asset_store.py is updated to use the new form.

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[jroachgolf84]

LGTM! Excellent addition :)

[amoghrajesh]

The failures are unrelated, merging