[AURON #2132] Add native support for instr function

[weimingdiit]

Which issue does this PR close?

Closes ##2132

Rationale for this change

To improve compatibility with Spark SQL string functions, we should support instr(str, substr) with Spark-compatible semantics.

Expected behavior

Function name: instr(str, substr)

Return value:

• Returns the 1-based index of the first occurrence of substr in str
• Returns 0 if substr is not found
• Returns 1 if substr is an empty string
• Returns NULL if either argument is NULL

Examples:

• instr('hello', 'll') -> 3
• instr('hello', 'x') -> 0
• instr('hello', '') -> 1

This behavior should match Spark SQL and work consistently for literal and expression inputs.

What changes are included in this PR?

This PR adds support for the Spark SQL instr() function in the native conversion path.
The following changes are included:

• Added StringInstr expression support in NativeConverters
• Mapped Spark instr(str, substr) to the existing native strpos scalar function
• Added regression tests to verify correctness for:
  • successful substring match
  • substring not found
  • empty substring handling
  • null input handling

Are there any user-facing changes?

NO.

How was this patch tested?

CI.