fix(e6): quote reserved-word keys in Databricks colon path access

[tkaunlaky-e6]

Problem

Databricks colon path access on a reserved-word key — e.g. Properties:limit — fails to transpile to valid E6. E6's colon-path grammar accepts only a non-reserved identifier after :, so a bare reserved word makes the E6/Calcite parser throw Encountered ":". The e6 generator only quoted RESERVED_DATATYPE_KEYWORDS (25 datatype names), leaving the other E6 reserved words (LIMIT, FROM, ORDER, …) bare.

This is the "Parse: Databricks colon accessor" failure bucket — 90 queries in the TomTom migration run, all Properties:limit.

Fix

Widen the colon-key quoting check in _render_variant_colon and json_extract_sql (databricks branch) from RESERVED_DATATYPE_KEYWORDS to also cover the full RESERVED_KEYWORDS set, emitting col:"limit". Non-reserved keys are unchanged (stay bare); datatype keys keep working.

Validation

• Verified against the live E6/Calcite engine: col:"limit" parses natively (0 transpiler fallbacks) while bare col:limit throws Encountered ":".
• SELECT Properties:limit → SELECT Properties:"limit"; the 13 non-reserved keys in the real query (country, area, custom_metrics_name, …) stay bare.
• Added test cases; full test_e6.py suite passes (59 tests / 896 subtests).

Example

-- Databricks
SELECT Properties:limit FROM t
-- e6 (before) -> SELECT Properties:limit    (rejected: Encountered ":")
-- e6 (after)  -> SELECT Properties:"limit"  (accepted natively)