feat(libsy): expose Python target bindings

[nachiketb-nvidia]

Stack

1. feat(libsy): expose typed Python protocol bindings #88 Protocol bindings
2. feat(libsy): expose Python target bindings #89 Target bindings (this PR)
3. feat(libsy): expose Python algorithm execution #87 Algorithm execution
4. feat(libsy): expose Python run streams #82 Host-driven run streams

Review after #88.

Why

Python needs a reusable way to describe the semantic targets consumed by Rust-owned routing algorithms without binding any algorithm yet.

What

• Bind LlmTarget and LlmTargetSet.
• Keep llm_client assignable before an algorithm snapshots the target set.
• Validate the presence of a callable call method.
• Expose only the target API and its stubs.
• Add focused target collection and mutation tests.

How

The Python objects retain target and client references. Conversion into Rust targets is deliberately added in the next PR, where algorithm execution first consumes them.

What to review

1. Target naming and lookup behavior.
2. Client assignment and validation.
3. The snapshot boundary reserved for algorithm construction.
4. Separation from the existing components LlmTarget.

Validation

• cargo fmt --all -- --check
• cargo clippy -p switchyard-py --all-targets -- -D warnings
• uv run pytest tests/test_libsy_protocol_bindings.py tests/test_libsy_target_bindings.py -q -o addopts= (9 passed)