build: bump ruff to 0.15.12 and enforce top-level imports

[hkad98]

Summary

Bumps ruff from 0.15.10 to 0.15.12 in uv.lock and adds PLC0415 (import-outside-top-level) to tool.ruff.lint.select to enforce the existing convention that imports live at the module top level.

Habitual lazy imports were lifted to module top in client.py, workspace_data_filters.py, environment.py, conftest.py, test_aac.py, test_catalog_user_service.py, test_tabbed_dashboard_translations.py, and test_bare_execution_response.py. # noqa: PLC0415 was added only where the lazy pattern is genuinely required:

• Optional dependencies guarded by try/except ImportError (github, deep_translator)
• Optional opentelemetry.exporter.* exporters declared as allowed-unresolved-imports
• scripts/docs/tests/* fixtures that import after monkeypatch.syspath_prepend

Test plan

JIRA: TRIVIAL
risk: low

[codecov]

Codecov Report

❌ Patch coverage is 37.50000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.96%. Comparing base (e53ef3e) to head (9c4b754).

Files with missing lines	Patch %	Lines
...r/src/gooddata_flight_server/utils/otel_tracing.py	0.00%	3 Missing ⚠️
...ckages/gooddata-dbt/src/gooddata_dbt/dbt_plugin.py	0.00%	1 Missing ⚠️
packages/gooddata-dbt/src/gooddata_dbt/utils.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1592   +/-   ##
=======================================
  Coverage   78.96%   78.96%           
=======================================
  Files         231      231           
  Lines       15578    15578           
=======================================
  Hits        12301    12301           
  Misses       3277     3277           

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