feat(googeai): add withBaseURL func option

[vladzabolotnyi]

Purpose

This PR introduces the WithBaseURL functional option to the Google AI provider. Currently, the provider defaults to the standard Google API endpoints; to use a custom URL, developers are forced to manipulate the low-level opts.ClientOptions slice directly.

This change provides a high-level, developer-friendly interface consistent with our implementations for Anthropic, OpenAI, and other LLM providers.

What's Done

• Added WithBaseURL(baseURL string) Option which utilizes option.WithEndpoint under the hood
• Added TestGoogleAIGenerateContentWithCustomBaseURL to verify the option is correctly applied

How Tested

Tested using the project's internal httprr integration testing framework.

PR Checklist

• Read the Contributing documentation.
• Read the Code of conduct documentation.
• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
• Describes the source of new concepts.
• References existing implementations as appropriate.
• Contains test coverage for new functions.
• Passes all golangci-lint checks.