refactor: Introduce SpillState enum for memory-limited NLJ execution (#21636)

## Which issue does this PR close?

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- Closes #.

## Rationale for this change

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The current approach does use Option fields as implicit state flags
(e.g., left_stream.is_some() to check if we're in memory-limited mode),
which is fragile. This introduces a dedicated enum to make the execution
state explicit.

## What changes are included in this PR?

Replace scattered Option fields and boolean flags with a dedicated
SpillState enum that explicitly tracks the fallback state:

- Disabled: fallback not possible (unsupported join type or no disk)
- Pending: fallback possible, holding left_plan and task_context for
re-execution on OOM
- Active: fallback triggered, holding all spill-related state
(left_stream, reservation, spill_manager, etc.)

This makes the state transitions explicit and eliminates the implicit
is_memory_limited() check based on Option::is_some().

Co-authored-by: Claude Code

## Are these changes tested?

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Existing tests

## Are there any user-facing changes?

No

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Author: viirya