fix: extend setuptools<82 retry to pyproject.toml apps

[socksy]

Summary

The legacy-setuptools retry added in #270 only kicked in for requirements.txt projects — pyproject.toml apps that needed pkg_resources (setuptools<82) at install time had no recovery path and just failed.

uv sync can't accept a CLI constraint, so the retry now drops out of uv sync into uv pip install with the project source (. for pyproject, -r requirements.txt otherwise) plus a setuptools<82 arg. Both kinds of project get the same fallback.

The call site in tower-runtime is now if res != 0 { retry } — no file-type knowledge — and the file-type branch lives inline in sync_with_legacy_setuptools_pin where the command args are built. should_use_legacy_setuptools_pin and spawn_requirements_install are gone; a small run_setup_child helper dedupes the stdout/stderr drain + wait between the initial install and the retry.

Test plan

• cargo test -p tower-uv --test sync_test (7 tests, including two new pyproject ones)
• Verified the new pyproject tests fail when production code is reverted (negative control)

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[sammuti]

LGTM