[Enhancement] Implementing dictification for ARRAY_AGG

[farhad-celo]

Why I'm doing:

The Low Cardinality Optimization (LCO) currently does not support ARRAY_AGG. When ARRAY_AGG operates on low-cardinality string columns that have global dictionaries, the aggregation processes full VARCHAR values instead of compact integer-encoded dictionary IDs. This means ARRAY_AGG misses the performance benefits (reduced memory, faster comparisons) of dictification.

What I'm doing:

Extends the Low Cardinality Optimization to support ARRAY_AGG with dict-encoded columns. When the value column (first argument) of ARRAY_AGG has a global dictionary, the optimization:

• Encodes the value and any dict-encodable ORDER BY columns as integer IDs throughout all aggregation stages.
• Computes the intermediate struct type with dict-encoded array fields (e.g. struct<ARRAY<INT>, ARRAY<INT>> instead of struct<ARRAY<VARCHAR>, ARRAY<VARCHAR>>)
• Correctly handles multi-stage aggregation by using the struct field mapping to independently determine which fields are dict-encoded in the merge stage
• Supports downstream operations on the dict-encoded result (e.g. UPPER(ARRAY_AGG(...)[1]) applies DictDecode at the projection level)

Key changes:

• DecodeContext.java: Refactored AggregateRewriter to handle functions with different intermediate types.
• DecodeCollector.java: Adds ARRAY_AGG to supported aggregate functions, gates on value column being a string type with a global dict, builds struct field mapping for intermediate types
• SessionVariable.java: New array_agg_low_cardinality_optimize session variable (default: true) gated behind existing array and struct LCO flags

Fixes #issue

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
  • This pr needs auto generate documentation
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 4.1
  • 4.0
  • 3.5
  • 3.4