feat(pathfinder): centralize CUDA env var handling, prioritize CUDA_PATH over CUDA_HOME

[rwgk]

Closes #1433

WIP-WIP-WIP

Current state: Squash-merge of PR #1519 rebased onto main. Untested (not even locally).

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[rwgk]

Decision: drop os.pathsep splitting of CUDA_HOME/CUDA_PATH

Both cuda_bindings/build_hooks.py and cuda_core/build_hooks.py currently split the env var value on os.pathsep and iterate over a list of CUDA roots. After reviewing the codebase and the broader ecosystem, I'm removing this pattern. Rationale:

• No real-world usage. The NVIDIA CTK installer (Windows), conda environments, pixi configs, and CMake's FindCUDAToolkit all set CUDA_PATH/CUDA_HOME to a single directory. There is no documented convention for pathsep-separated multi-path values.
• Never tested. The two build_hooks.py files even disagreed on env var priority (CUDA_HOME > CUDA_PATH in bindings, CUDA_PATH > CUDA_HOME in core) — if anyone had used multi-path, that inconsistency would have surfaced.
• Avoids premature complexity. Adding multi-path support to the new centralized get_cuda_home_or_path() (especially combined with the conflict-warning logic) would add design surface for a feature nobody uses today.

Both _get_cuda_paths() functions will be replaced with a plain get_cuda_home_or_path() call returning a single path. If a real multi-path use case surfaces later, we can add it with proper design at that point.

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Related commit: 2164c33

[rwgk]

Decision: treat empty CUDA_PATH/CUDA_HOME as undefined

Rob's original implementation preserved empty strings (CUDA_PATH="") as valid return values from get_cuda_home_or_path(). I'm changing this so that empty strings are equivalent to the variable not being set. Rationale:

• No valid use case. An empty string is not a valid filesystem path. Nobody intentionally sets CUDA_PATH="" to mean "the CUDA path is empty."
• Fixes a subtle shadowing bug. With the old behavior, CUDA_PATH="" + CUDA_HOME=/usr/local/cuda would return "" (CUDA_PATH wins by priority), silently hiding the valid CUDA_HOME. Treating empty as undefined lets it correctly fall through.
• Simplifies callers. All downstream consumers already do if not cuda_path: raise .... Returning "" just pushes the falsy check to every call site.
• Matches convention. The standard os.environ.get("CUDA_PATH") idiom throughout the codebase has always treated empty and unset interchangeably (via if not value). This aligns with that expectation.

This addresses my review comment on Rob's PR: #1519 (comment)