Consolidate dialect duplication and make mypy strict pass cleanly

[richardwooding]

Summary

Two behavior-preserving maintainability passes. The 728 unit tests are unchanged and pass, generated SQL is byte-identical, and mypy --strict now reports 0 errors (was 171).

1. Dialect de-duplication

The six dialect files repeated many byte-identical write_* methods, so any change had to be hand-applied to every copy.

• Moved clear-majority shared methods to the Dialect base class as concrete defaults: write_string_literal, write_string_concat (||), write_duration, write_interval, write_timestamp_arithmetic, write_struct_close, and write_type_name (via a TYPE_MAP class attribute). Dialects override only where they genuinely diverge (e.g. BigQuery string escaping, MySQL/Spark CONCAT, SQLite interval modifiers).
• Added validate_sql_field_name() in _utils shared by DuckDB/BigQuery/MySQL/SQLite (parameterized by reserved set + length + dialect label).
• Refactored convert_re2_to_posix to reuse _validate_regex_common (dropped ~60 duplicated validation lines).
• Extracted field_is_json() (in schema.py), shared by _converter and _analysis.

Net duplicate-function clusters dropped 20 → 15 (remaining ones are 2–3-dialect minority overlaps, left explicit by design).

2. mypy strict clean (171 → 0)

All 171 errors were typing gaps, not runtime bugs.

• Parametrized lark generics: Tree[Token] (via a TreeT alias), list[...], Interpreter[Token, None].
• Modeled child nodes as Branch = Tree[Token] | Token to match lark's Tree.children, widening child-consuming helpers and adding a small _children() helper. isinstance(..., Tree) runtime checks were left untouched (parametrized generics can't be used in isinstance).
• Fixed six one-offs: replaced two tuple-expression lambdas with named def ... -> None helpers, renamed a float-branch variable, added a missing return annotation.
• Dropped the "pre-existing lark type errors are expected" note from CLAUDE.md.

Test plan

• uv run pytest tests/ --ignore=tests/integration → 728 passed
• uv run mypy src/pycel2sql/ → Success, no issues (was 171 errors)
• uv run ruff check src/ tests/ → clean
• Integration tests (tests/integration/) deferred to CI — no local container runtime available.

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