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 54.54545% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.16%. Comparing base (db61d59) to head (8696769).

Files with missing lines	Patch %	Lines
...o/query/parser/MongoGroupTypeExpressionParser.java	40.00%	2 Missing and 1 partial ⚠️
.../core/documentstore/parser/GroupByAliasGetter.java	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #327   +/-   ##
=========================================
  Coverage     81.16%   81.16%           
- Complexity     1679     1690   +11     
=========================================
  Files           243      243           
  Lines          7762     7769    +7     
  Branches        769      771    +2     
=========================================
+ Hits           6300     6306    +6     
+ Misses          967      966    -1     
- Partials        495      497    +2     

Flag	Coverage Δ
integration	81.16% <54.54%> (+<0.01%)	⬆️
unit	58.73% <54.54%> (+0.23%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[github-actions]

Test Results

  128 files  +1    128 suites  +1   38s ⏱️ +5s
  890 tests +1    889 ✅ +1  1 💤 ±0  0 ❌ ±0 
1 260 runs  +1  1 259 ✅ +1  1 💤 ±0  0 ❌ ±0 

Results for commit 8696769. ± Comparison against base commit db61d59.

♻️ This comment has been updated with latest results.