Enable multiple pipe_input targets

[Probablism]

Fixes #1177

This enables multiple-target pipe_input behavior and restores the corresponding targeting API and tests.

Changes

• Allow pipe_input(..., on_input=[...]) to target more than one function parameter.
• Restore step(...).on_input(...) for per-step input targeting.
• Support mixed global/local targeting, where local targets add to the global target list.
• Reject ambiguous or invalid targets instead of falling back to the first parameter.
• Update the pipe_input docstring and restore the matching unit/resource tests.

How I tested this

• UV_CACHE_DIR=/tmp/hamilton-uv-cache .uv-tools/bin/uv run pytest tests/function_modifiers/test_macros.py -q -k pipe_input
• UV_CACHE_DIR=/tmp/hamilton-uv-cache .uv-tools/bin/uv run pytest tests/function_modifiers/test_macros.py -q
• UV_CACHE_DIR=/tmp/hamilton-uv-cache .uv-tools/bin/uv run pytest tests/function_modifiers tests/test_function_modifiers.py -q
• UV_CACHE_DIR=/tmp/hamilton-uv-cache .uv-tools/bin/uv run pytest tests/ --cov=hamilton --ignore tests/integrations -q
• UV_CACHE_DIR=/tmp/hamilton-uv-cache .uv-tools/bin/uv run ruff check hamilton/function_modifiers/macros.py tests/function_modifiers/test_macros.py tests/resources/pipe_input.py
• UV_CACHE_DIR=/tmp/hamilton-uv-cache .uv-tools/bin/uv run ruff format --check hamilton/function_modifiers/macros.py tests/function_modifiers/test_macros.py tests/resources/pipe_input.py
• git diff --check

Notes

This keeps the old default behavior for on_input=None and the existing single-target behavior for on_input="param". I did not change pipe_output or mutate.

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 focused checks listed above & 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.