fix(db2): escape SQL identifiers and literals in metadata/DDL paths (#1914)

[HandSonic]

Summary

Fixes the DB2 plugin portion of #1914 (34 SQL-injection findings across database plugins). All identifier/literal interpolation in the DB2 plugin's metadata, DDL-builder, and export paths now goes through a new Db2SqlEscapes helper (single-quote doubling for string literals, double-quote doubling + wrapping for delimited identifiers), and free-form inputs that cannot be safely escaped (column default values, length units, index sort direction, fallback column types) are validated against strict whitelists.

Changed paths:

• DB2MetaData: tableDDL (db2look token call), indexes, view, function, procedure, getMetaDataName
• DB2DBManager: exportTable/Views/Procedures/Triggers, connectDatabase (SET SCHEMA), dropTable, copyTable, truncateTable
• DB2SqlBuilder: buildCreateTable/buildAlterTable/buildCreateSchema incl. comments, plus new buildDropTable/buildTruncateTable overrides (SPI defaults interpolated raw names)
• DB2ColumnTypeEnum: column name escaping; strict DEFAULT value pattern; length-unit whitelist; safe fallback for sized types like VARCHAR(10) that miss the exact enum map (previously delegated to raw SPI buildDefaultColumn — the one exploitable path found in adversarial review)
• DB2IndexTypeEnum: index/constraint names and comments escaped; ASC/DESC exact-match validation

Two defense-in-depth measures beyond plain escaping:

• tableDDL rejects schema/table names containing ", because the names are embedded in a double-quoted db2look option string (-td "%s" -t "%s") where quote-doubling is not a valid neutralization.
• All regex validators are anchored with \A/\z (Java $ matches before a trailing newline).

Trade-offs

• The strict DEFAULT pattern (\A[A-Za-z0-9_+/: \t.-]+\z) rejects quoted string literals such as 'abc'; string defaults remain available via the existing EMPTY_STRING / NULL keywords. This was an explicit reviewer request to prevent literal-context toggling inside enclosing CREATE/ALTER statements.
• The fallback column-type whitelist accepts only LETTERS, LETTERS(n), LETTERS(n,n); anything else throws IllegalArgumentException instead of being interpolated.
• truncateTable emits TRUNCATE TABLE "x" without IMMEDIATE (matches the SPI shape; DB2 LUW may require IMMEDIATE in some contexts — functional nit, not a security issue).

Residual risk

• SPI IColumnBuilder.buildDefaultColumn still concatenates name/type raw; the DB2 plugin no longer calls it, but other plugins still do. Out of scope for this PR (plugin-level fixes only, to keep the per-plugin PRs independent); tracked separately.
• buildPageLimit / buildExplain concatenate the user's own SQL by design (SQL-editor flow), unchanged.
• The db2look option string is parsed inside IBM's SYSPROC.DB2LK_GENERATE_DDL; JDBC-level injection is closed (single quotes doubled) and " is rejected up front, but the procedure's internal option parsing is not auditable here.

Test evidence

• Db2SqlEscapesTest: 23/23 pass on a clean build (mvn -pl chat2db-community-plugins/chat2db-community-db2 test), covering every changed path with real attack strings: x'; DROP TABLE t; --, bad"name, 0; DROP TABLE t, OCTETS) DROP TABLE t, 1), (evil INT DEFAULT (0, x' OR '1'='1, trailing-newline bypasses (0\n, VARCHAR(10)\n, OCTETS\n, ASC\n), db2look option injection (x" -t "y), and sized-type fallback (VARCHAR(10)).
• Two independent adversarial review rounds (including live throwaway attack probes against compiled classes, 13 payloads) ended in PASS; every probe was either contained inside a quoted identifier/literal or rejected with IllegalArgumentException.

[HandSonic]

Converting to draft: final adversarial review pass is still in progress for this branch. Will mark ready for review once it completes. (process note: PR was opened prematurely by automation)