fix(snowflake): null-safe metadata compare in column-type build

[Aias00]

Related issue

Closes #1998

Summary

SnowflakeColumnTypeEnum.buildDataType and buildNullable compared
Integer-typed column metadata (columnSize, decimalDigits,
nullable) using .equals(). These fields are boxed Integer and are
null when the driver omits metadata — the same null-metadata scenario
already handled in DBStructUtils (#1968) but not applied here.

• buildDataType (lines 128–129) dereferenced
  column.getOldColumn().getColumnSize() / getDecimalDigits() → NPE on MODIFY.
• buildNullable (line 174) called column.getNullable().equals(...)
  before its own getNullable() != null guard (line 175) → NPE on MODIFY
  when nullable metadata is missing.

Switched both to Objects.equals(...), and added a regression test
(SnowflakeColumnTypeEnumTest) that constructs a MODIFY column with null
Integer metadata on both the current and old column and asserts
buildCreateColumnSql no longer throws.

Affected surfaces

• Frontend / Web
• Backend / API / Storage
• Database plugin / Driver
• JCEF / Desktop packaging
• CI / Build / Release
• Documentation only

Verification

• Commands and results:

  mvn -B -f chat2db-community-server/pom.xml \
    -pl chat2db-community-plugins/chat2db-community-snowflake -am \
    -Dmaven.test.skip=false -DskipTests=false \
    -Dtest=ai.chat2db.plugin.snowflake.enums.type.SnowflakeColumnTypeEnumTest \
    -Dsurefire.failIfNoSpecifiedTests=false -Dmaven.test.failure.ignore=false test
  

  → Tests run: 2, Failures: 0, Errors: 0, Skipped: 0; reactor BUILD SUCCESS.
• Manual verification: N/A (no Snowflake instance available locally); the test reproduces the previously-throwing MODIFY path with null metadata.
• UI evidence: N/A

Risk and compatibility

• Public API or stored data: N/A — only changes generated ALTER text.
• Database or driver compatibility: Snowflake MODIFY-column generation no longer NPEs on columns whose size/scale/nullable metadata is absent; behavior for populated metadata is unchanged (Objects.equals matches .equals semantics for non-null).
• Network, privacy, or security: N/A.
• Community / Local / Pro boundary: N/A.
• Backward compatibility: Only fixes a crash; no API change.

Reviewer map

• Start here: SnowflakeColumnTypeEnum.java lines 128–130 and 175 — .equals → Objects.equals; new import java.util.Objects.
• Failure condition: MODIFY-column ALTER generation throws NPE when column size/scale/nullable metadata is missing.
• Rollback or disable path: Revert this single commit; the new test documents the expected behavior.

Contributor declaration

• I linked the Issue that defines this change.
• I tested the affected behavior and reported the actual results above.
• I did not include credentials, private data, or generated build output.
• I disclosed substantial AI assistance below, or this PR contains no substantial AI-generated code.

AI assistance: The fix, test, verification, and PR description were produced with Claude Code assistance.

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[openai0229]

Reviewed and updated. The null-safe size/scale comparisons are correct. I also tightened the MODIFY nullable path so missing metadata cannot be misinterpreted as a NOT NULL change, and added a regression assertion for the generated SQL. Targeted Snowflake tests pass locally.