Fix: widen confidence type in legacy HTTP API

[leeclemnet]

What does this PR do?

Fix for v3 workflow blocks 422 errors on remote execution path only: https://roboflow.slack.com/archives/C06BD36S37A/p1777316040643609

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Testing

• I have tested this change locally
• I have added/updated tests for this change

Test details:

New unit test: test legacy http API with string-valued confidence modes

Integration testing:

• Force self._step_execution_mode = ExecutionMode.REMOTE in v3 instance seg workflow block
• Run local inference server forcing v0 API via

docker run --rm -p 9001:9001 \
    -e ROBOFLOW_API_KEY=${ROBOFLOW_API_KEY} \
    -e WORKFLOWS_REMOTE_API_TARGET=hosted \
    -e HOSTED_INSTANCE_SEGMENTATION_URL=http://localhost:9001 \
    -e LOG_LEVEL=DEBUG \
    --gpus=all \
    --name inference-test \
    roboflow/roboflow-inference-server-gpu:dev

• Reproduced 422 error on main with confidence=best

HTTPCallErrorError(description='422 Client Error: Unprocessable Entity for url: http://localhost:9001/bouldering-holds-9wavr-wrfb4/2?api_key=EW***Ap&confidence=best&mask_decode_mode=accurate&tradeoff_factor=0.0&max_detections=300&overlap=0.3&disable_active_learning=True&source=workflow-execution', api_message='N/A',status_code=422)

• Verified this PR fixes the 422 error and workflow succeeds with model block confidence="best"

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in hard-to-understand areas
• My changes generate no new warnings or errors
• I have updated the documentation accordingly (if applicable)

Additional Context

[leeclemnet]

rebased branch onto tag v1.2.5 for branch release + deploy

[leeclemnet]

Rebased back onto main for merge