feat: BigQuery Storage v1beta1 API migration guide

[benhxy]

Description

Fixes b/505001153. BigQuery Storage team is planning to deprecate v1beta1 API. This PR adds a migration guide from v1beta1 to v1.

Checklist

Testing

• I have tested this change on a live environment and verified it works as intended.

Compliance & Style

• I have followed the Sample Guidelines.
• The README is updated with all relevant information (if applicable).
• If this PR adds a new sample directory: I updated the CODEOWNERS file.

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Post-Approval Actions

• Please merge this PR for me once it is approved

[gemini-code-assist]

Code Review

This pull request adds a migration guide for transitioning Python code using the BigQuery Storage API from version v1beta1 to v1. The review feedback correctly identifies a critical issue in the guide's text and code examples: the parameter for specifying a stream in read_rows is incorrectly documented as name instead of read_stream, which would cause a runtime TypeError for users following the guide.

[gemini-code-assist]

In the Python client library for v1, the parameter name for read_rows is read_stream, not name. Using name will result in a TypeError at runtime.

Please update this description to refer to read_stream instead of name.

[gemini-code-assist]

The keyword argument for specifying the stream in read_rows is read_stream, not name. Passing name will raise a TypeError: read_rows() got an unexpected keyword argument 'name'.

Suggested change

	# read_rows accepts stream name and offset directly.
	# Note that the parameter name in the library helper is 'name', which maps to 'read_stream' in proto.
	reader = client.read_rows(
	name=session.streams[0].name,
	offset=0
	)
	# read_rows accepts stream name and offset directly.
	reader = client.read_rows(
	read_stream=session.streams[0].name,
	offset=0
	)