Combining `tools` with `responseFormat` on Gemini is documented as unsupported but not validated

On the Gemini provider it is documented that tools cannot be combined with `responseFormat` and that only one of the two should be configured. Currently a flow that sets both is accepted by the editor and at save time, and the failure happens at the provider:

```
Function calling with a response mime type: 'application/json' is unsupported
```

Neither `tools` nor `responseFormat` is named in that message, so the documented rule has to be found before the cause can be understood. The same error was reported in https://github.com/kestra-io/plugin-ai/issues/217, which was closed by documenting the constraint. On the Anthropic provider, a configured response format is already rejected before the call, so there is precedent for the check.

Minimal example:

```yaml
id: tools_and_response_format
namespace: company.team

tasks:
- id: agent
type: io.kestra.plugin.ai.agent.AIAgent
prompt: Which flow should be executed to restart the database?
provider:
type: io.kestra.plugin.ai.provider.GoogleGemini
modelName: gemini-3.5-flash
apiKey: "{{ secret('GEMINI_API_KEY') }}"
configuration:
responseFormat:
type: JSON
jsonSchema:
type: object
properties:
decision:
type: string
tools:
- type: io.kestra.plugin.ai.tool.KestraFlow
namespace: company.team
flowId: hello-world
description: Say hello
```

## To do

- A flow that configures both `tools` and `responseFormat` on a provider where the two are incompatible should be rejected with a message naming both properties and the provider.
- The rejection should happen at validation time so the conflict is visible in the editor before the flow is executed.