Add describe_system_view to table client

[vgvoleg]

Closes #744.

Adds TableClient.describe_system_view (sync and async) — a stateless TableService.DescribeSystemView RPC that returns a full description of a system view through a new SystemViewSchemeEntry (system view id, name, columns, primary key, attributes). The DescribeTable* messages are deprecated for system views, so this provides the dedicated description.

The proto is already present in the vendored stubs. The RPC carries no session id, so it goes directly through the driver like bulk_upsert (no session pool).

Verified end-to-end against a trunk YDB (every /local/.sys/* view describes correctly, missing paths raise SchemeError); unit tests plus sync and async integration tests added.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.33%. Comparing base (9143705) to head (5499658).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #868      +/-   ##
==========================================
+ Coverage   82.28%   82.33%   +0.05%     
==========================================
  Files          99       99              
  Lines       12724    12749      +25     
  Branches     1242     1242              
==========================================
+ Hits        10470    10497      +27     
+ Misses       1800     1798       -2     
  Partials      454      454              

Flag	Coverage Δ
integration	80.21% <96.00%> (+0.04%)	⬆️
unit	48.37% <92.00%> (+0.26%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
ydb/_apis.py	100.00% <100.00%> (ø)
ydb/_session_impl.py	61.11% <100.00%> (+1.17%)	⬆️
ydb/aio/table.py	80.32% <100.00%> (+0.10%)	⬆️
ydb/table.py	66.69% <100.00%> (+0.38%)	⬆️

... and 1 file with indirect coverage changes

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[Copilot]

Pull request overview

Adds first-class support for describing YDB system views via the dedicated TableService.DescribeSystemView RPC, exposing it as a new TableClient.describe_system_view API (sync and async) that returns a new SystemViewSchemeEntry.

Changes:

• Add describe_system_view to the sync/async table clients and wire it to the stateless DescribeSystemView RPC (no session pool).
• Introduce SystemViewSchemeEntry and response/request wrapping in _session_impl.
• Add unit + integration tests (sync + async) and a CHANGELOG entry.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
ydb/table.py	Adds describe_system_view and introduces SystemViewSchemeEntry used to represent system view schema.
ydb/table_test.py	Adds unit tests for request factory and response wrapper behavior (success + error).
ydb/aio/table.py	Adds async describe_system_view wrapper for the aio table client.
ydb/_session_impl.py	Implements request factory and response wrapper for DescribeSystemView.
ydb/_apis.py	Registers DescribeSystemView method name constant for the table service.
tests/table/test_table_client.py	Adds sync integration test covering success and SchemeError on missing view.
tests/aio/test_table_client.py	Adds async integration test covering success and SchemeError on missing view.
CHANGELOG.md	Documents the new public API addition.

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[vgvoleg]

Addressed both review comments in a2d20e4:

• SystemViewSchemeEntry.attributes now stores the raw protobuf map, consistent with TableSchemeEntry.
• Documented describe_system_view / SystemViewSchemeEntry in docs/table.rst (sync + async examples) and docs/apireference.rst.