feat: add Gemini model provider

[notgitika]

Description

This PR adds the Gemini model provider support to Strands

Related Issues

Documentation PR

TBD -- will edit description once its out

Type of Change

New feature

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• pre-commit run --all-files
• I ran hatch run prepare
• hatch test -- can verify all tests pass
• hatch run test-integ -- did not get a lot of useful information out of this (most tests skipped because of missing auth keys of those providers) but I did run my own integ test file for gemini that I added hatch test tests_integ/models/test_model_gemini.py. They all pass

Note to reviewer: re: integ tests -- a couple of the integ tests I added for Gemini can get flaky because of the nature of the tests, so I set the temperature to 0.15. They can still fail intermittently. If we do not feel the need to include a bunch of these tests and only include a few to be consistent with the other model provider integ tests, please let me know

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly -- adding Gemini in the user guide in the docs repository is TBD and a fast follow
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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