refactor(toolchain): replace resolved_venv_toolchain with venv_exec_toolchain_type

[gregmagolan]

Summary

resolved_venv_toolchain was a workaround: a regular dep with cfg = "exec" used to pull the venv binary into exec config for the build action in py_venv.bzl. This PR replaces it with a proper toolchain type, and takes the opportunity to clean up the overall toolchain design.

Changes

PrebuiltToolConfig redesign — cfg is now mandatory and accepts "target", "exec", or "both". Tools with cfg = "both" (like venv) register under two toolchain types from a single downloaded binary: target_compatible_with for runfiles (runs on the user's machine) and exec_compatible_with for build actions (runs on the exec host). The struct exposes toolchain_type and exec_toolchain_type fields derived from cfg; callers no longer pass types explicitly.

TOOL_CFGS simplified — venv_exec entry removed; venv now carries cfg = "both". The separate py/private/toolchain/venv_exec/ directory is gone.

source_toolchain simplified — takes only name and bin; derives everything else from TOOL_CFGS. Creates {name}_tool / {name}_source_toolchain for target-cfg tools and {name}_exec_tool / {name}_exec_source_toolchain for exec-cfg tools.

Toolchain type renames for consistency:

• unpack_toolchain_type → unpack_exec_toolchain_type
• target_exec_toolchain_type → native_build_toolchain_type (self-documenting: forces exec platform to match target for native wheel builds)
• TARGET_EXEC_TOOLCHAIN → NATIVE_BUILD_TOOLCHAIN

New — VENV_EXEC_TOOLCHAIN constant and venv_exec_toolchain_type toolchain type for the exec-cfg venv binary used by py_venv build actions.

py_venv.bzl — replaces ctx.attr._venv (private dep with cfg = "exec") with ctx.toolchains[VENV_EXEC_TOOLCHAIN]. Removes resolved_venv_toolchain rule and target entirely.

repo.bzl — _toolchains_repo_impl emits target_compatible_with or exec_compatible_with toolchain declarations based on which types are present, with no string-interpolated cfg values. _prebuilt_tool_repo_impl downloads once per tool regardless of how many toolchain types it serves.

[aspect-workflows]

Bazel 8 (Test)

All tests were cache hits

111 tests (100.0%) were fully cached saving 53s.

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Bazel 9 (Test)

All tests were cache hits

111 tests (100.0%) were fully cached saving 1m 20s.

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Bazel 8 (Test)

e2e

All tests were cache hits

51 tests (100.0%) were fully cached saving 55s.

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Bazel 9 (Test)

e2e

All tests were cache hits

51 tests (100.0%) were fully cached saving 43s.

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Bazel 8 (Test)

examples/uv_pip_compile

All tests were cache hits

1 test (100.0%) was fully cached saving 444ms.

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