Allow Mongo group-by on an aliased function expression

[bhubam]

Summary

• Add an optional alias on FunctionExpression so a function can be used as a Mongo group key.
• When the alias is set, $addFields materializes the function and $group groups by that field. A blank alias still fails with the existing unsupported-operation error.
• Covers the actor interval bucket (INTERVAL_START_TIME), which is floor(lastActivity / interval) * interval.

Merge order

Merge this before the entity-service interval change. Entity-service will pick up FunctionExpression.alias through a BOM upgrade; this PR does not publish a version.

Test plan

• MongoFunctionGroupByTest asserts $addFields for INTERVAL_START_TIME, $group._id references that field, and the projection reads $_id.INTERVAL_START_TIME.
• CI on this PR.

[codecov]

Codecov Report

❌ Patch coverage is 89.65517% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.23%. Comparing base (db61d59) to head (3952cbd).

Files with missing lines	Patch %	Lines
...o/query/parser/MongoGroupTypeExpressionParser.java	85.00%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #327      +/-   ##
============================================
+ Coverage     81.16%   81.23%   +0.06%     
- Complexity     1679     1698      +19     
============================================
  Files           243      243              
  Lines          7762     7780      +18     
  Branches        769      771       +2     
============================================
+ Hits           6300     6320      +20     
+ Misses          967      962       -5     
- Partials        495      498       +3     

Flag	Coverage Δ
integration	81.23% <89.65%> (+0.06%)	⬆️
unit	58.84% <89.65%> (+0.34%)	⬆️

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Test Results

  128 files  +1    128 suites  +1   44s ⏱️ +11s
  894 tests +5    893 ✅ +5  1 💤 ±0  0 ❌ ±0 
1 264 runs  +5  1 263 ✅ +5  1 💤 ±0  0 ❌ ±0 

Results for commit 3952cbd. ± Comparison against base commit db61d59.

♻️ This comment has been updated with latest results.

[suddendust]

Two cases:

1. What happens today if the function alias is null? Can you add a test case for this?
2. What happens when the function alias and the selection alias are different? Smth like:

IdentifierExpression timestamp =
    IdentifierExpression.of("attributes.last_activity_timestamp.value.long");
ConstantExpression interval = ConstantExpression.of(86_400_000L);

FunctionExpression bucket =
    FunctionExpression.builder()
        .alias("bar")                     
        .operator(MULTIPLY)
        .operand(
            FunctionExpression.builder()
                .operator(FLOOR)
                .operand(
                    FunctionExpression.builder()
                        .operator(DIVIDE)
                        .operand(timestamp)
                        .operand(interval)
                        .build())
                .build())
        .operand(interval)
        .build();

Query query =
    Query.builder()
        .addSelection(bucket, "foo")      
        .addSelection(AggregateExpression.of(COUNT, IdentifierExpression.of("id")), "count")
        .addAggregation(bucket)
        .build();

[bhubam]

Null or blank function alias still throws UnsupportedOperationException (Grouping by a function ... is not yet supported). No $addFields stage. Covered by rejectsFunctionGroupByWhenAliasIsMissing (null, "", and " ").

If the function alias and the selection alias differ (bar vs foo):

$addFields writes the bucket as bar
$group._id is { bar: "$bar" }
the projection is { foo: "$_id.bar" }
When both aliases are the same (INTERVAL_START_TIME), the pipeline is unchanged. See 3952cbd.