Gate Gemini image cassette tests on image feature

[gold-silver-copper]

Summary

• Gates the Gemini image-generation cassette module behind #[cfg(feature = "image")].
• Adds a CI default-feature test-target compile step: cargo test -p rig --tests --no-run.
• This prevents CI from only validating the --all-features graph and missing root integration-test compile failures when optional features are disabled.

Why CI passed before

• The CI test job ran cargo nextest run --all-features --retries 2.
• The Gemini image-generation test module compiled under --all-features because the image feature was enabled, so rig::image_generation, ImageGenerationClient, and GEMINI_2_5_FLASH_IMAGE existed.
• A plain cargo test uses the default feature set, where image is disabled; the ungated Gemini image-generation test module still compiled and referenced image-only APIs, causing the failure.
• The new CI no-run step compiles the root integration-test targets with default features before the all-features nextest run.

Validation

• cargo fmt --check
• cargo test -p rig --tests --no-run
• cargo test -p rig --test gemini -- --nocapture --test-threads=1
• cargo test -p rig --all-features --test gemini image_generation -- --nocapture --test-threads=1
• cargo clippy --all-targets --all-features
• cargo test was also run. The prior Gemini image-generation compile failure is fixed; the run later fails on an unrelated existing OpenAI cassette replay mismatch in openai::cassette::permission_control::permission_control_streaming_example.

Notes

• No cassette fixtures changed.
• The Gemini test gating mirrors existing image-feature gating used by OpenAI, xAI, HuggingFace, and other image-generation provider tests.