feat: Support left expression

[Shekharrajak]

Closes #3181

Rationale for this change

Enable native execution for Spark's LEFT(str, len) function to improve query performance by avoiding JVM fallback.

What changes are included in this PR?

• Added CometLeft serializer that transforms LEFT(str, len) to Substring(str, 1, len) protobuf
• Registered Left expression in QueryPlanSerde string expressions map
• No protobuf or Rust changes required (reuses existing Substring implementation)

How are these changes tested?

Added 4 test suites in CometExpressionSuite:

• Basic functionality (various lengths: 0, -1, positive, exceeds length)
• Unicode character handling (emoji, multi-byte chars)
• Equivalence verification with SUBSTRING(str, 1, len)
• Dictionary encoding preservation

[andygrove]

@Shekharrajak can you run make to update the generated configs.md

[andygrove]

LGTM pending CI. Thanks @Shekharrajak