feat(exasol): transpile USE/OPEN SCHEMA and SHOW TABLES to system-table query

[mkcorneli]

Summary

Two related Exasol statement-transpilation features:

1. USE <schema> ↔ OPEN SCHEMA <schema> — Exasol uses OPEN SCHEMA where other dialects use USE. Before this PR, parsing OPEN SCHEMA test raised a ParseError, and transpiling USE test from MySQL produced a literal USE test which Exasol rejects.
2. SHOW TABLES [FROM <schema>] → SELECT on SYS.EXA_ALL_TABLES — Exasol has no native SHOW TABLES. Before this PR the base generator emitted an empty string, leaving callers with no query to run.

Before

>>> sqlglot.transpile("USE test", read="mysql", write="exasol")[0]
'USE test'                                 # Exasol rejects

>>> sqlglot.parse_one("OPEN SCHEMA test", dialect="exasol")
ParseError: Invalid expression / Unexpected token. Line 1, Col: 16.

>>> sqlglot.transpile("SHOW TABLES FROM test", read="mysql", write="exasol")[0]
''                                         # base generator marks SHOW unsupported

After

>>> sqlglot.transpile("USE test", read="mysql", write="exasol")[0]
'OPEN SCHEMA test'

>>> sqlglot.transpile("OPEN SCHEMA test", read="exasol", write="exasol")[0]
'OPEN SCHEMA test'

>>> sqlglot.transpile("SHOW TABLES FROM test", read="mysql", write="exasol")[0]
\"SELECT TABLE_NAME FROM SYS.EXA_ALL_TABLES WHERE TABLE_SCHEMA = 'TEST'\"

>>> sqlglot.transpile("SHOW TABLES", read="mysql", write="exasol")[0]
'SELECT TABLE_NAME FROM SYS.EXA_ALL_TABLES WHERE TABLE_SCHEMA = CURRENT_SCHEMA'

Implementation notes

• `use_sql` rejects non-schema kinds. For `USE ROLE admin` or `USE WAREHOUSE wh` (Snowflake forms), the generator emits an unsupported warning and falls back to the base `USE` output — it does not silently rewrite them as `OPEN SCHEMA admin`, which would be semantically wrong.
• `show_sql` falls through for other SHOW kinds. Only `SHOW TABLES` is rewritten; every other kind (SHOW DATABASES, SHOW ENGINES, etc.) falls through to the base `show_sql` which marks them unsupported as before — so there is no semantic regression for any kind we don't explicitly handle.
• Schema literal is uppercased. Exasol stores identifiers in `EXA_ALL_TABLES` uppercased, so `SHOW TABLES FROM test` becomes `... WHERE TABLE_SCHEMA = 'TEST'` to match.
• `OPEN SCHEMA` parsing. `OPEN` has no dedicated `TokenType`, so the parser override intercepts it via `_match_text_seq("OPEN", "SCHEMA")` in `_parse_statement` and falls through to the base parser for everything else.

Test plan

• New `test_use_to_open_schema` covers basic form, quoted identifier, Exasol round-trip, and `USE ROLE admin` unsupported path
• New `test_show_tables` covers `SHOW TABLES FROM schema` and `SHOW TABLES` (CURRENT_SCHEMA fallback)
• Full `tests/dialects/` suite runs cleanly (no new failures vs. main)
• `ruff check` + `ruff format --check` pass

[georgesittas]

Thanks! Can you please rebase off of main and resolve conflicts?