Auto-infer BigQuery schema from schema'd PCollections for FILE_LOADS

[nikitagrover19]

Fixes #28162

What

WriteToBigQuery with method=FILE_LOADS previously required an explicit
schema, even when the input PCollection already carried a Beam schema - for example a PCollection produced by
ReadFromBigQuery(..., output_type='BEAM_ROW'), or any PCollection of
NamedTuples, dataclasses, or Beam Rows. The STORAGE_WRITE_API method
already auto-infers a schema in this situation; this brings FILE_LOADS
(both JSON and AVRO temp file formats) in line with that behavior.

How

• Added beam_schema_to_bq_table_schema() in bigquery_schema_tools.py,
  the reverse of the existing generate_user_type_from_bq_schema(). It
  converts a Beam schema_pb2.Schema into a BigQuery TableSchema dict,
  handling atomic types, logical types (Timestamp, Date, Decimal),
  nested/repeated fields, and nullability, and raises a clear ValueError
  for types with no BigQuery equivalent (e.g. MapType).
• WriteToBigQuery.expand()'s FILE_LOADS branch now tries
  schema_from_element_type(pcoll.element_type) when self.schema is None, before the existing "schema required for AVRO" check, so AVRO
  loads benefit from inference too, not just JSON.
• Falls back to the prior behavior (schema stays None) when the
  PCollection has no attached Beam schema (e.g. plain dicts), so this is
  purely additive and non-breaking.

Testing

• Unit tests for the schema conversion covering atomic types, timestamps,
  decimals, nullable fields, repeated fields, nested records, repeated
  records, and the unsupported-MapType error case
  (bigquery_schema_tools_test.py).
• Integration-style tests on WriteToBigQuery confirming: schema
  auto-inference for JSON file loads, schema auto-inference for AVRO file
  loads (previously always raised "A schema must be provided" here),
  and that plain-dict PCollections are unaffected
  (bigquery_test.py).
• Ran the existing TestWriteToBigQuery test class locally to check for
  regressions; all passing except one pre-existing test that requires
  real GCP credentials unavailable in my local environment (unrelated to
  this change).

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