feat: Introduce shallow query engine

[ferenc-csaky]

Key Changes

• Add support for shallow query engines, which can generate queries but cannot serve database-level API requests.
• Reclassify Snowflake as a shallow query engine.
• Validate server configurations require a compatible full query engine.
• Add integration coverage for Snowflake and query-engine validation scenarios.

[codecov]

Codecov Report

❌ Patch coverage is 2.56410% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 18.28%. Comparing base (d4123f8) to head (fca0f75).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...a/com/datasqrl/engine/pipeline/SimplePipeline.java	0.00%	15 Missing ⚠️
...atasqrl/config/QueryEngineConfigConverterImpl.java	0.00%	7 Missing ⚠️
...ase/relational/AbstractJDBCShallowQueryEngine.java	0.00%	6 Missing ⚠️
...er/src/main/java/com/datasqrl/util/EngineUtil.java	0.00%	5 Missing ⚠️
...src/main/java/com/datasqrl/config/JdbcDialect.java	0.00%	2 Missing ⚠️
...asqrl/engine/database/relational/DuckDBEngine.java	0.00%	2 Missing ⚠️
...rl/engine/database/relational/SnowflakeEngine.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2244      +/-   ##
============================================
- Coverage     18.28%   18.28%   -0.01%     
  Complexity     1133     1133              
============================================
  Files           612      613       +1     
  Lines         17828    17836       +8     
  Branches       2175     2177       +2     
============================================
+ Hits           3260     3261       +1     
- Misses        14246    14253       +7     
  Partials        322      322              

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