Add SerializedVariableInterval for deadline alerts

[SameerMesiah97]

Description

This change is a follow-up for PR #64751 that introduces a core-side SerializedVariableInterval representation for dynamically configured deadline intervals.

VariableInterval remains the SDK-facing type used in DAG definitions, while deserialization converts it to SerializedVariableInterval for core deadline evaluation. Variable-backed interval resolution is moved to this serialized counterpart.

Rationale

Deadline references already use dedicated serialized representations to maintain a separation between SDK and core types. Providing the same representation for VariableInterval prevents the SDK dataclass from propagating beyond the deserialization boundary and allows core deadline evaluation to operate on a dedicated core-side type.

Notes

• Variable-backed intervals now support zero and negative values to align with existing timedelta semantics, where intervals represent offsets from the deadline reference and are not required to be positive.
• A stale comment in test_dagrun_deadline_variable_interval_stable has been updated.

Tests

Moved the existing VariableInterval.resolve() tests to SerializedVariableInterval.resolve() to reflect that interval resolution is now performed by the core-side representation.

The resolution tests cover valid Variable values, including zero and negative offsets, as well as missing and invalid Variable values.

Backwards Compatibility

VariableInterval remains the SDK-facing type for configuring dynamic deadline intervals, so existing DAG definitions continue to work unchanged.

The change only affects the internal representation after deserialization and where Variable-backed intervals are resolved. Existing timedelta intervals and serialized deadline behavior remain unchanged.

[SameerMesiah97]

@amoghrajesh @ferruzzi

Hopefully, this follow-up PR should address the concerns you raised on PR #64751.