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

[andygrove]

@Shekharrajak could you take a look at the CI failures?

[Shekharrajak]

@Shekharrajak could you take a look at the CI failures?

Updated configs.md now the checks should succeed. Please trigger the workflow

[Shekharrajak]

Looks like order was not correct last time.

Moved Left.enabled config entry to come after LastDay.enabled - in configs.md

[codecov-commenter]

Codecov Report

❌ Patch coverage is 10.52632% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.99%. Comparing base (f09f8af) to head (476e04a).
⚠️ Report is 864 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/main/scala/org/apache/comet/serde/strings.scala	5.55%	17 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3206      +/-   ##
============================================
+ Coverage     56.12%   59.99%   +3.86%     
- Complexity      976     1418     +442     
============================================
  Files           119      170      +51     
  Lines         11743    15793    +4050     
  Branches       2251     2610     +359     
============================================
+ Hits           6591     9475    +2884     
- Misses         4012     4998     +986     
- Partials       1140     1320     +180     

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[Shekharrajak]

Checks are looking fine now.

[andygrove]

Thanks again @Shekharrajak