[Feature] Implement variant type casting functions

[xxubai]

Why I'm doing:

What I'm doing:

1. Support casting variant type to primitive types, decimal, array, struct, map and JSON.
2. Introduce some query functions: get_variant_string, get_variant_bool, get_variant_int, get_variant_double etc.

Will add some documents in the following PRs

What type of PR is this:

• [ ] BugFix
• [x] Feature
• [ ] Enhancement
• [ ] Refactor
• [ ] UT
• [ ] Doc
• [ ] Tool

Does this PR entail a change in behavior?

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

If yes, please specify the type of change:

• [x] 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 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
  • [ ] 4.0
  • [ ] 3.5
  • [ ] 3.4
  • [ ] 3.3

---

[!NOTE] Adds comprehensive VARIANT casting (to numeric, boolean, string, decimal, JSON, ARRAY/MAP/STRUCT) and new query helpers get_variant_{bool,int,double,string}.

• Casting/Type System:

  • Implement casting from VARIANT to primitives (BOOLEAN/INT/FLOAT/DOUBLE/STRING), decimals (DECIMAL32/64/128), JSON, and complex types (ARRAY/MAP/STRUCT).
  • Add CastVariantToArray, CastVariantToMap, CastVariantToStruct expressions and integrate into VectorizedCastExprFactory.
  • Extend decimal cast pipeline to handle VARIANT -> DECIMAL*.
  • Update string/JSON cast paths to accept VARIANT and disable JIT for VARIANT casts.
  • FE: adjust TypeManager and PrimitiveType implicit/compatibility rules for VARIANT (including complex types) and JSON interactions.

• Variant utilities:

  • Introduce variant_converter for casting Variant to target types; expose cast_variant_value_to helpers.
  • Enhance VariantPathParser::seek validation and Parquet VariantMetadata::get_index binary search logic.

• Functions/APIs:

  • Add builtins: get_variant_bool, get_variant_int, get_variant_double, get_variant_string; wire prepare/close and codegen entries.
  • Extend VariantFunctions to support typed queries via shared path parsing and casting.

• Build/Plumbing:

  • Register new sources in CMake; update function registry (gensrc/script/functions.py).

^{Written by Cursor Bugbot for commit 888c1afb383441823e9ff10776483674a4c7e088. This will update automatically on new commits. Configure here.}

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Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[github-actions[bot]]

[Java-Extensions Incremental Coverage Report]

:white_check_mark: pass : 0 / 0 (0%)

[github-actions[bot]]

[FE Incremental Coverage Report]

:white_check_mark: pass : 14 / 14 (100.00%)

file detail

	path	covered_line	new_line	coverage	not_covered_line_detail
:large_blue_circle:	com/starrocks/sql/common/TypeManager.java	12	12	100.00%	[]
:large_blue_circle:	com/starrocks/connector/iceberg/IcebergApiConverter.java	2	2	100.00%	[]

[github-actions[bot]]

[BE Incremental Coverage Report]

:x: fail : 366 / 500 (73.20%)

file detail

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:large_blue_circle:	be/src/exprs/cast_expr.cpp	135	193	69.95%	[195, 261, 262, 267, 268, 275, 276, 277, 559, 648, 672, 696, 1159, 1160, 1224, 1408, 1776, 1777, 1778, 1779, 1817, 1818, 1819, 1820, 1821, 1822, 1825, 1826, 1827, 1828, 1829, 1830, 1837, 1838, 1839, 1858, 1859, 1860, 1871, 1872, 1873, 1874, 1889, 1892, 2001, 2005, 2006, 2007, 2008, 2011, 2012, 2013, 2014, 2018, 2019, 2020, 2021, 2022]
:large_blue_circle:	be/src/exprs/cast_expr_map.cpp	54	64	84.38%	[101, 102, 123, 124, 147, 148, 149, 150, 151, 152]
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:large_blue_circle:	be/src/exprs/cast_expr_struct.cpp	51	55	92.73%	[136, 172, 193, 205]