[spark] Support ON_ERROR = CONTINUE / SKIP_FILE in COPY INTO

[JunRuiLee]

Motivation

This is part of #8005.

COPY INTO previously only supported ON_ERROR = ABORT_STATEMENT: any parse or
cast error aborted the entire command. In production data-loading pipelines a
single malformed row or file would then fail the whole batch, which is often
too strict. This adds two error-tolerant modes:

• CONTINUE — skip bad rows and load the rest (row-level tolerance).
• SKIP_FILE — skip any file that contains an error, all-or-nothing per file.

ABORT_STATEMENT remains the default, so existing behavior is unchanged.

Changes

• Grammar: ON_ERROR now accepts CONTINUE and SKIP_FILE in addition to
  ABORT_STATEMENT.
• Result schema gains two columns:
  • errors_seen (BIGINT) — number of error rows per file.
  • first_error (STRING) — first error message, NULL when the file is clean.
  • status now also reports PARTIALLY_LOADED and LOAD_FAILED.
• Error detection runs once per batch; both modes write in a single commit.
  Load history is recorded so error-tolerant runs stay idempotent under
  FORCE = FALSE.
• Refactor: CopyIntoTableExec is split into focused helpers
  (CopyIntoHelper, CopyIntoCastValidator, CopyIntoDataFrameBuilder,
  CopyIntoErrorHandler, CopyIntoResultBuilder), shared across CSV/JSON/Parquet.
• Docs updated in sql-write.md, including the CSV column-count-mismatch caveat
  under CONTINUE.

Supported for CSV, JSON, and Parquet.