feat(duckdb): Add transpilation support for SUBSTR-SUBSTRING functions

[fivetran-amrutabhimsenayachit]

This PR aims to resolve the following issues:
Issue 1 — Zero start position
Snowflake SUBSTR(str, 0, n) treats 0 as 1. DuckDB returns a shorter string. Fix: clamp 0 → 1.

Issue 2 — Negative length
Snowflake SUBSTR(str, pos, -n) returns ''. DuckDB silently returns wrong data. Fix: clamp negative → 0.

python3 -c 'import sqlglot; print(sqlglot.transpile("SELECT SUBSTR('"'"'testing 1 2 3'"'"', 9, 3) AS basic, SUBSTR('"'"'testing 1 2 3'"'"', 9) AS no_len, SUBSTR('"'"'testing 1 2 3'"'"', 0, 3) AS zero_start, SUBSTR('"'"'testing 1 2 3'"'"', -5, 3) AS neg_start, SUBSTR(NULL, 1, 3) AS null_str, SUBSTR('"'"'testing'"'"', 1, 0) AS zero_len, SUBSTR('"'"'testing 1 2 3'"'"', 5, -3) AS neg_len", read="snowflake", write="duckdb")[0])' | duckdb
┌─────────┬─────────┬────────────┬───────────┬──────────┬──────────┬─────────┐
│  basic  │ no_len  │ zero_start │ neg_start │ null_str │ zero_len │ neg_len │
│ varchar │ varchar │  varchar   │  varchar  │ varchar  │ varchar  │ varchar │
├─────────┼─────────┼────────────┼───────────┼──────────┼──────────┼─────────┤
│ 1 2     │ 1 2 3   │ tes        │ 1 2       │ NULL     │          │         │
└─────────┴─────────┴────────────┴───────────┴──────────┴──────────┴─────────┘

[github-actions]

SQLGlot Integration Test Results

Comparing:

• this branch (sqlglot:RD-1147762-substr-substring, sqlglot version: RD-1147762-substr-substring)
• baseline (main, sqlglot version: 0.0.1.dev1)

By Dialect

dialect	main	sqlglot:RD-1147762-substr-substring	transitions	links
bigquery -> bigquery	22925/22930 passed (100.0%)	21255/21255 passed (100.0%)	No change	full result / delta
bigquery -> duckdb	1304/1674 passed (77.9%)	0/0 passed (0.0%)	Results not found	full result / delta
snowflake -> duckdb	1521/2678 passed (56.8%)	1521/2678 passed (56.8%)	No change	full result / delta
snowflake -> snowflake	65927/65927 passed (100.0%)	65927/65927 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: 107722 total, 106190 passed (pass rate: 98.6%), sqlglot version: 0.0.1.dev1

sqlglot:RD-1147762-substr-substring: 104373 total, 103216 passed (pass rate: 98.9%), sqlglot version: RD-1147762-substr-substring

Transitions:
No change

Dialect pair changes: 0 previous results not found, 1 curent results not found

✅ 17 test(s) passed