Refactor(schema): cache find lookups in schema

[georgesittas]

Benched this using annotate_types on a query with 2,400 columns references across 8 tables (8x60 cols, each referenced 5x).

before (main): 35.9ms median, 33.7ms min
after (cache): 33.3ms median, 32.0ms min (~7% faster)

The gain is modest in wall-time because the trie walk itself is cheap. The main benefit is eliminating 2,400 redundant base find() calls down to the number of unique (table, ensure_data_types) pairs.

[github-actions]

SQLGlot Integration Test Results

Comparing:

• this branch (sqlglot:jo/avoid_unnecessary_schema_lookups, sqlglot version: jo/avoid_unnecessary_schema_lookups)
• baseline (main, sqlglot version: 0.0.1.dev1)

By Dialect

dialect	main	sqlglot:jo/avoid_unnecessary_schema_lookups	transitions	links
bigquery -> bigquery	24647/24652 passed (100.0%)	23497/23497 passed (100.0%)	No change	full result / delta
bigquery -> duckdb	867/1154 passed (75.1%)	0/0 passed (0.0%)	Results not found	full result / delta
duckdb -> duckdb	5823/5823 passed (100.0%)	0/0 passed (0.0%)	Results not found	full result / delta
snowflake -> duckdb	1129/1935 passed (58.3%)	0/0 passed (0.0%)	Results not found	full result / delta
snowflake -> snowflake	65133/65133 passed (100.0%)	63027/63027 passed (100.0%)	No change	full result / delta
databricks -> databricks	1370/1370 passed (100.0%)	1370/1370 passed (100.0%)	No change	full result / delta
postgres -> postgres	6042/6042 passed (100.0%)	6042/6042 passed (100.0%)	No change	full result / delta
redshift -> redshift	7101/7101 passed (100.0%)	7101/7101 passed (100.0%)	No change	full result / delta

Overall

main: 113210 total, 112112 passed (pass rate: 99.0%), sqlglot version: 0.0.1.dev1

sqlglot:jo/avoid_unnecessary_schema_lookups: 101037 total, 101037 passed (pass rate: 100.0%), sqlglot version: jo/avoid_unnecessary_schema_lookups

Transitions:
No change

Dialect pair changes: 0 previous results not found, 3 current results not found

✅ 43 test(s) passed