[Enhancement] Stable StarRocks Driver and Add test code

[EdwardArchive]

Related PR : #8454

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Problem:

• VARCHAR columns were returned as []byte instead of string due to MySQL
  driver behavior when scanning into any type
• DECIMAL columns returned as float64, causing precision loss for large values

Solution:

• Add starrocksRows wrapper with custom MapScan() using sql.NullXXX types
• DECIMAL now returns as string to preserve full precision (same as MySQL driver)
• Add prepareScanDest() for type-safe scan destinations

Testing:

• Add Docker-based integration tests using StarRocks 4.0.3 all-in-one container
• 29 test cases covering all StarRocks data types:
  • Basic types (BOOLEAN, INT, BIGINT, FLOAT, DOUBLE, VARCHAR, etc.)
  • Complex types (ARRAY, MAP, STRUCT)
  • Aggregate types (HLL, BITMAP)
  • High-precision DECIMAL (DECIMAL32, DECIMAL64, DECIMAL128)
  • Unicode strings (Korean, Chinese, Japanese, Emoji)
  • Boundary values and NULL handling
• Refactor teststarrocks to read from init.sql file for better maintainability

Type mapping (MySQL protocol):

• BOOLEAN → int16 (reported as TINYINT)
• DECIMAL → string (precision preserved)
• LARGEINT → string (128-bit exceeds int64)
• ARRAY/MAP/STRUCT → string (JSON format)
• JSON → string (reported as VARCHAR)

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[EdwardArchive]

These test failures are due to missing environment variables (S3, GCS, Druid, MotherDuck credentials)