feat: prepare client-side delegation and telemetry for pandas-gbq

[shuoweil]

Injects pandas-gbq/<version> into the client user-agent when query result conversion to DataFrame / GeoDataFrame is executed and supported by pandas-gbq.

This PR enables backend metrics and telemetry to distinguish between client-side conversion and delegated conversions without altering existing DataFrame output or behavior.

Key Changes

• Telemetry Injection: Appends pandas-gbq/<version> to ClientInfo.user_agent in RowIterator.to_dataframe() and to_geodataframe().
• Idempotency: Ensures the user-agent token is not duplicated across multiple calls on the same client connection.
• Graceful Fallback: Handles missing dependencies or unsupported environments without raising errors.

Fixes #<540939659> 🦕

[gemini-code-assist]

Code Review

This pull request introduces query delegation support to the pandas-gbq package when retrieving dataframes via to_dataframe. It adds version helper utilities to check if delegation is supported, issues a deprecation warning if the core client fallback is used, and delegates the dataframe creation to pandas_gbq when available. The review feedback suggests making the pandas-gbq import and version checks more robust by catching general exceptions rather than just ImportError, using getattr to safely retrieve the package version, and ensuring that pyarrow is skipped in tests when not installed to prevent test failures.

[gemini-code-assist]

Catching only ImportError when importing pandas_gbq can lead to unhandled exceptions if the import fails due to other issues (e.g., compiled extension load failures, AttributeError, or TypeError from broken dependencies). Additionally, packaging.version.parse can raise InvalidVersion if the version string is malformed. Catching Exception ensures a robust fallback to version 0.0.0.

Suggested change

	if self._installed_version is None:
	try:
	import pandas_gbq # type: ignore
	self._installed_version = packaging.version.parse(
	getattr(pandas_gbq, "__version__", "0.0.0")
	)
	except ImportError:
	self._installed_version = packaging.version.parse("0.0.0")
	if self._installed_version is None:
	try:
	import pandas_gbq # type: ignore
	self._installed_version = packaging.version.parse(
	getattr(pandas_gbq, "__version__", "0.0.0")
	)
	except Exception:
	self._installed_version = packaging.version.parse("0.0.0")

[shuoweil]

I do not want to hide errors for users.

[gemini-code-assist]

Similar to installed_version, catching only ImportError when importing pandas_gbq can lead to unhandled exceptions if the import fails due to other issues. Catching Exception ensures a robust fallback to 0.

Suggested change

	if self._delegation_api_version is None:
	try:
	import pandas_gbq # type: ignore
	self._delegation_api_version = getattr(
	pandas_gbq, "_internal_delegation_api_version", 0
	)
	except ImportError:
	self._delegation_api_version = 0
	if self._delegation_api_version is None:
	try:
	import pandas_gbq # type: ignore
	self._delegation_api_version = getattr(
	pandas_gbq, "_internal_delegation_api_version", 0
	)
	except Exception:
	self._delegation_api_version = 0

[shuoweil]

I do not want to hide errors for users.

[parthea]

Converting to draft until presubmits are green

[sycai]

Is this PR a functional duplicate of #17957? They look quite similar.