Add OTel spans to model artifact loading

[ecarrara]

What does this PR do?

Adds OpenTelemetry child spans inside the existing model.load span to make model artifact loading observable. Previously, model.load_time_seconds was a single opaque measurement — it was impossible to tell whether the time was spent reading from local disk cache or downloading weights from a remote source.

Three new spans are emitted, each carrying model.id and model.artifacts.source attributes:

Span	model.artifacts.source	Triggered by
model.artifacts.load	local_cache	weights already in disk cache
model.artifacts.download	s3	download from S3 bucket
model.artifacts.download	roboflow_api	download from Roboflow API

Related Issue(s): N/A

Type of Change

• New feature (non-breaking change that adds functionality)

Testing

• I have added/updated tests for this change

Test details: Spans are emitted via the existing start_span helper which degrades to a noop when OTel is not configured, so no existing behaviour changes.

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

model.load_time_seconds on the parent model.load span is intentionally kept. It measures total model initialization time, which includes ONNX session creation and other setup beyond artifact fetching. The new child spans cover only the artifact portion, so the full picture looks like:

model.load  (total: model.load_time_seconds)
├── model.artifacts.load / model.artifacts.download  (artifact fetch)
└── [remaining time = ONNX session init, etc.]

[hansent]

LGTM