feat(py): core support for background model operations and typed model refs

[cabljac]

First slice of the #5806 split: FinishReason.ABORTED (typing + genkit-schema.json), a cancel-operation action kind with registry lookup, background operation lifecycle updates, generate_operation/check_operation plumbing, and a config_schema parameter on model_ref.

Stack (split of #5806)

1. feat(py): core support for background model operations and typed model refs #5854 core support for background model operations and typed model refs
2. feat(py/plugins/google-genai): raw Interactions HTTP client, converters, and options #5855 Interactions HTTP client, converters, and options
3. feat(py/plugins/google-genai): Deep Research, Antigravity, and Lyria interaction models #5856 Deep Research, Antigravity, and Lyria interaction models
4. refactor(py/plugins/google-genai): move Veo to background model actions #5857 move Veo to background model actions
5. feat(py/plugins/google-genai): wire Interactions models into GoogleAI plugin #5858 wire Interactions models into GoogleAI plugin
6. feat(py/plugins/google-genai): finish_message for image models and effective config echo #5859 gemini finish_message (parallel, based on this PR)

Recombining this stack reproduces the tree of #5806 byte-for-byte. Work co-authored with @huangjeff5.

Breaking changes

• ModelRequest.config no longer coerces bare dict configs into ModelConfig: ModelRequest(config={...}) keeps the dict, and ModelRequest[PluginConfig] coerces into the plugin schema instead. Code that relied on attribute access on an implicitly-coerced ModelConfig must use dict access or bind a config schema.
• ModelRequest.config now raises TypeError for values that are neither a BaseModel nor a dict.

Cross-language files

The aborted finish reason already exists in the JS runtime on main; this PR syncs the tools zod source (genkit-tools/common/src/types/model.ts), the generated genkit-schema.json, and Go's generated go/ai/gen.go to match (CI enforces all three). No hand-written Go or tools changes.

[gemini-code-assist]

Code Review

This pull request enhances background model support and model configuration validation in the Python SDK. Key changes include adding the 'aborted' finish reason, allowing background models to return an Operation to poll, improving model resolution with helpful namespace hints, and refining ModelRequest config validation to gracefully handle plugin-specific schemas and dicts. The feedback suggests optimizing the input validation path in _action.py to avoid the overhead of catching ValidationError on every model call by checking and dumping BaseModel inputs upfront.