[Python] Support Lakehouse runtime catalog tables in ReadFromBigQuery…

[claudevdm]

… DIRECT_READ

Port of the Java change in #39597 to the Python SDK.

• parse_table_reference now assigns segments by explicit splitting (mirroring BigQueryHelpers.parseTableSpec) so 4-part project.catalog.namespace.table and project:catalog.namespace.table specs map to a composite catalog.namespace dataset id. Previously the dotted form raised and the colon form silently bound the project to project:catalog.
• Storage Read split tolerates tables that report no numBytes by requesting zero streams so the Read API picks the stream count; the export source's size estimate returns 0 instead of raising.
• Unit tests for the new spec forms, domain-scoped projects, invalid specs, and the numBytes-missing paths.

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[tvalentyn]

Since this is new functionality, can we force a standard format without dot/colon ambiguity and without potentially ambiguous parsing (project.dataset.table vs [no_project].catalog.namespace.table)? For example we could change the docstring to:

TableReference can be a PROJECT:DATASET.TABLE, PROJECT.DATASET.TABLE, DATASET.TABLE, or PROJECT.CATALOG.NAMESPACE.TABLE (for Lakehouse catalog tables).

[claudevdm]

I dont see a way we can force a standard format.

table is just a string, so we only find out whether it's a 3-part or 4-part
reference by parsing it.

We also can't drop the colon form. get_hashable_destination converts a
TableReference into PROJECT:DATASET.TABLE as the canonical string representation.

So we need colon because of get_hashable_destination, and dot because it's the
modern SQL form we want to encourage. Deprecating colon would be nice, but it's the
destination key throughout the write path, so that's a bigger breaking change.

Updated the docs to mention that project is required for pcnt format

[codecov]

Codecov Report

❌ Patch coverage is 97.95918% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 58.67%. Comparing base (9685063) to head (91aac7e).
⚠️ Report is 30 commits behind head on master.

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/io/gcp/bigquery_tools.py	97.36%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #40225      +/-   ##
============================================
+ Coverage     58.65%   58.67%   +0.02%     
  Complexity    13843    13843              
============================================
  Files          2579     2581       +2     
  Lines        270366   271356     +990     
  Branches      11212    11212              
============================================
+ Hits         158587   159224     +637     
- Misses       105747   106100     +353     
  Partials       6032     6032              

Flag	Coverage Δ
python	79.66% <97.95%> (-0.20%)	⬇️

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[tvalentyn]

thanks!