fix(serve): propagate additional_model_data_sources for JumpStart models

[e-davidson]

ModelBuilder._build_for_jumpstart() copied image_uri, env,
enable_network_isolation, model_reference_arn, and model_data from the
resolved JumpStart init_kwargs, but never propagated
additional_model_data_sources. Any JumpStart model whose spec declares
an extra S3 data channel (e.g. an EAGLE speculative decoding draft
model, a LoRA adapter, or a tokenizer override) therefore had that
channel silently dropped from the CreateModel call, and the container
failed at runtime trying to read artifacts from a path that was never
mounted.

Propagate additional_model_data_sources from init_kwargs onto the
builder so _prepare_container_def_base attaches it to the CreateModel
request. Each source is filtered to only the keys the CreateModel API
accepts (ChannelName, S3DataSource); JumpStart specs may include extra
metadata keys such as HostingEulaKey that the API rejects.

Adds unit tests for both the propagation+filtering path and the no-op
case when the spec declares no additional data sources.

Issue #, if available:

Description of changes:

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