feat: lazy imports gapic v1

[hebaalazzeh]

Description

This PR initiates the rollout of our PEP810 lazy-loading architecture to google-api-core, starting with the gapic_v1 module.

By applying Python 3.15's native explicit lazy imports (PEP 0810), we defer the eager execution of inner modules (like method.py and config.py) which recursively pull in massive third-party C-extensions like grpcio. This acts as the first step in addressing the high initialization latency and peak memory footprints we're seeing on Serverless cold-starts.

Older Python runtimes (3.14 and below) safely ignore the __lazy_modules__ set and fall back to standard eager execution, meaning this introduces zero backwards compatibility risk.

Related Documents

• Design Doc: go/sdk:api-core-lazy-loading
• Original GAPIC Generator Implementation PR: #17591

Note: This PR was kept intentionally small for review speed. We will be executing an iterative rollout, extending this pattern to other heavy modules like exceptions and operations_v1 in fast follow-up PRs once this structural pattern is approved.

[gemini-code-assist]

Code Review

This pull request adds an import_profile test target to the CI script and attempts to implement lazy loading for gapic_v1 submodules using Python 3.15's __lazy_modules__ feature. However, a critical issue was identified: placing the imports inside a conditional block (sys.version_info < (3, 15)) prevents them from being executed at all on Python 3.15+, which will lead to AttributeErrors at runtime. To fix this, the imports should remain at the top level so that Python 3.15+ can lazily load them while older versions load them eagerly.