fix(contrib/drivers/dm): preserve quoted identifiers

[yuyinw]

Description

This PR fixes the DM driver SQL filter so it preserves double-quoted identifiers, which are used by DM as identifier delimiters.

Previously, DoFilter removed all double quotes from SQL, breaking quoted table or column names. The filter now keeps explicit quoted identifiers and preserves the original SQL whitespace while still applying the existing DM-specific function/keyword conversions.

The DM metadata lookup now first keeps the existing uppercase lookup behavior, then falls back to the original table-name case so quoted, case-sensitive table names can still resolve their fields. It also trims DM identifier quotes before building metadata table-name and owner-name candidates.

Testing

• go test -run 'TestDriverDoFilterPreservesDoubleQuotedIdentifiers|TestDriverDoFilterQuotesOnlyUnquotedIndexKeyword|TestDriverGetCharsDisablesAutomaticIdentifierQuoting|TestTableNameCandidatesForMetadata|TestSchemaNameForMetadata' -count=1
• go test -run '^$' -count=1

[Copilot]

Pull request overview

This PR updates the DM (contrib/drivers/dm) database driver to better support DM’s use of double quotes (") as identifier delimiters, avoiding accidental removal of quoted identifiers and improving metadata lookups for case-sensitive (quoted) table names.

Changes:

• Adjust DM driver quoting behavior by disabling automatic identifier quoting via GetChars() and preserving double-quoted identifiers in DoFilter.
• Improve TableFields metadata resolution by trying uppercase table names first, then falling back to the original case.
• Add focused unit tests covering DoFilter, GetChars, and the new metadata table-name candidate logic.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
contrib/drivers/dm/dm.go	Changes DM driver quote chars behavior by returning empty chars from GetChars().
contrib/drivers/dm/dm_do_filter.go	Stops stripping " from SQL and refines INDEX quoting to avoid touching quoted/string-literal occurrences.
contrib/drivers/dm/dm_do_filter_test.go	Adds unit tests for quoted identifier preservation, selective INDEX quoting, and GetChars().
contrib/drivers/dm/dm_table_fields.go	Adds table-name candidate fallback logic for metadata lookups and reuses chosen candidate for PK queries.
contrib/drivers/dm/dm_table_fields_test.go	Adds a unit test for tableNameCandidatesForMetadata.
contrib/drivers/dm/dm_z_unit_init_test.go	Updates test DB setup/teardown helpers to handle more table-name variants (including quoted forms).

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