[mmotoyama]

Fixes b/573773536.

When --transpile translates a field or metric expression to
BigQuery SQL, transpileModel previously wrote the result only to
the in-memory expression field and skipped ANSI_SQL expressions.
This meant serialized models lost the translated BigQuery SQL, and
ANSI_SQL expressions that wrap column names in double quotes
("orders"."amount", which BigQuery reads as string literals) were
left untranslated.

This PR updates SQL translation in three ways:

• Store translated SQL as a BIGQUERY entry in dialects (and set
  stringForm to false so bare-string expressions expand into a
  dialects list when serialized) rather than overwriting
  expression. Because translated SQL now lives in dialects,
  validateBigQueryMetrics and measureProblem in validate.ts
  read expressionForDialect(metric, 'BIGQUERY') before falling
  back to metric.expression.
• Translate ANSI_SQL expressions that double-quote column names
  when no BIGQUERY entry is present, in addition to vendor-only
  expressions. hasDoubleQuotedColumn detects double-quoted column
  names by comparing how the SQL parser reads the expression in
  ANSI_SQL (where both "..." and `...` are quoted column
  identifiers) versus BIGQUERY (where only `...` is).
• Align POLYGLOT_DIALECTS with PARSER_DIALECTS in
  sql_parser.ts, adding SPANNER (as BigQuery), ALLOYDB (as
  PostgreSQL), and SQLSERVER (as T-SQL), and removing dialect
  names the loader no longer accepts.

[feixufeixu]

The three required comments need changes before this merges. The other two are optional.

[feixufeixu]

Approving. Please apply the required comments before you merge.