fix: Enable explode() and flatten() table functions in local testing …

[JoshElkind]

fix: Enable explode() and flatten() table functions in local testing mode
This addresses GitHub issue #3565 (SNOW-2213161) where calling explode() on a DataFrame in local testing mode failed with:
AttributeError: 'MockSelectStatement' object has no attribute 'snowflake_plan'

Root Cause:

MockSelectStatement lacked the snowflake_plan property that table_function.py expects for schema inference.
The mock execution engine had no implementation for the built-in FLATTEN table function (used internally by explode).

Changes:

Added snowflake_plan property to MockSelectable (select_statement.py) as an alias for execution_plan. Includes detailed comment explaining why this alias is safe and what could cause future divergence.

Implemented handle_flatten_function() in plan.py with:
Clear documentation of supported vs unsupported parameters
NotImplementedError for PATH and RECURSIVE parameters (not supported)

Support for: input, outer, mode parameters
Array flattening (VALUE column)
Object/dict flattening (KEY + VALUE columns)
Added comprehensive tests (13 total) covering:
Basic explode with arrays, integers, maps
Schema verification
explode_outer with empty/null arrays
Column preservation
Direct flatten() usage
NotImplementedError for unsupported parameters
Verification that non-flatten UDTFs still work

Design Decisions:

This is a MINIMAL implementation for explode() compatibility, NOT a complete FLATTEN implementation. Unsupported features raise clear errors.

Early-return pattern ensures existing UDTF code path is unchanged.
SEQ, PATH, INDEX, THIS output columns are not implemented (use integration tests if your tests require these).
Testing:
468 mock tests pass (13 new)
1759 unit tests pass

Performance: ~65ms for 1000 rows (acceptable for local testing)
Which Jira issue is this PR addressing?

Fixes SNOW-2213161

Fill out the following pre-review checklist:
[x] I am adding a new automated test(s) to verify correctness of my new code
[ ] If this test skips Local Testing mode, I'm requesting review from @snowflakedb/local-testing
[ ] I am adding new logging messages
[ ] I am adding a new telemetry message
[ ] I am adding new credentials
[ ] I am adding a new dependency
[x] If this is a new feature/behavior, I'm adding the Local Testing parity changes.
[x] I acknowledge that I have ensured my changes to be thread-safe. Follow the link for more information: Thread-safe Developer Guidelines
[ ] If adding any arguments to public Snowpark APIs or creating new public Snowpark APIs, I acknowledge that I have ensured my changes include AST support. Follow the link for more information: AST Support Guidelines
Please describe how your code solves the related issue.
The fix addresses two gaps in the local testing mock framework:
Problem 1: Missing API compatibility
When explode() is called, table_function.py accesses select_statement.snowflake_plan.output to infer column types. MockSelectStatement didn't have this property, causing the AttributeError.
Solution: Added snowflake_plan as a property alias to execution_plan in MockSelectable. This is safe because MockExecutionPlan.output provides the same List[Attribute] interface that SnowflakePlan.output does.
Problem 2: No FLATTEN implementation
explode() internally uses Snowflake's built-in FLATTEN table function. The mock framework only handled user-defined table functions (UDTFs), not built-ins.
Solution: Added handle_flatten_function() that executes FLATTEN logic locally:
For arrays: expands each element into a row with VALUE column
For objects/dicts: expands each key-value pair into rows with KEY and VALUE columns
Supports outer=True for null/empty handling (used by explode_outer)
Raises NotImplementedError for unsupported parameters (path, recursive) to prevent silent behavior drift
Isolation: The fix uses an early-return pattern in handle_udtf_expression() so existing UDTF handling is completely unchanged. A dedicated test verifies custom UDTFs still work after this change.

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[JoshElkind]

I have read the CLA Document and I hereby sign the CLA

[JoshElkind]

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