Upgrade Datadog plugin to ddtrace 4.x public API

[Dev-iL]

related: #1288
fixes: #1562

ddtrace 3.0 removed Span and context from the top-level package. Migrate to the canonical ddtrace.trace module, bump the version constraint to >=4.0, fix stale sf-hamilton package references, and add smoke tests for the plugin.

Summary

• Migrate h_ddog.py imports from removed top-level ddtrace.Span/ddtrace.context to the canonical ddtrace.trace module (ddtrace.trace.Span, ddtrace.trace.Context)
• Bump version constraint from ddtrace<3.0 to ddtrace>=4.0 in both the datadog optional dependency and the docs dependency group
• Fix stale sf-hamilton package references to apache-hamilton in the ImportError message and DDOGTracer docstring
• Remove the outdated TODO comment about ddtrace 3.0 deprecation
• Add smoke tests for the plugin (tests/plugins/test_h_ddog.py)

Motivation

ddtrace 3.0 removed Span and context from the top-level ddtrace package (DataDog/dd-trace-py#12186). The Hamilton plugin was pinned to ddtrace<3.0 as a temporary workaround. This PR migrates to the stable public API at ddtrace.trace, which is the canonical location in ddtrace 4.x.

Changes

File	What changed
hamilton/plugins/h_ddog.py	from ddtrace import Span, context, tracer split into from ddtrace import tracer + from ddtrace.trace import Context, Span; context.Context replaced with Context in type annotations and constructor calls; TODO comment removed; sf-hamilton references updated to apache-hamilton
pyproject.toml	ddtrace<3.0 changed to ddtrace>=4.0 in [project.optional-dependencies].datadog and docs
tests/plugins/test_h_ddog.py	New file with 7 tests covering span lifecycle (happy path, error path, task-based), AsyncDDOGTracer instantiation, and serialization/deserialization round-trip

No breaking changes

The public API surface of DDOGTracer and AsyncDDOGTracer is unchanged: same constructor signatures, same methods, same base class hierarchy. Only internal import paths changed.

Test plan

• DDOGTracer span lifecycle: before_graph -> before_node -> after_node -> after_graph
• Error propagation through span __exit__
• Task-based span lifecycle with before_task/after_task
• AsyncDDOGTracer instantiation with all parameter combinations
• __getstate__/__setstate__ serialization round-trip
• _serialize_span_dict/_deserialize_span_dict round-trip
• Module imports successfully with ddtrace 4.4.0
• No stale ddtrace<3.0 pins or sf-hamilton references remain

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.