uv fails to find workspace root for nested packages

### Summary

In a multi-level workspace, uv appears to treat nested members differently from top-level members:
```
mkdir -p foo/a/b
cd foo; uv init
cd a; uv init
cd b; uv init

cd ../ # we're under 'foo/a' now
uv sync # creates .venv and uv.lock at the workspace root 'foo/'

cd b
uv sync # creates a new .venv and uv.lock under 'foo/a/b'
```

I would expect that `foo` workspace member `a/b` continues to use the workspace-level .venv/uv.lock as it did for the first-level workspace member `a`.

`uv sync -v --all-packages` from within the `foo/a` folder:
```
DEBUG uv 0.9.7 (Homebrew 2025-10-30)
DEBUG Acquired shared lock for `/Users/stewartadam/.cache/uv`
DEBUG Found project root: `/Users/stewartadam/foo/a`
DEBUG Found workspace root: `/Users/stewartadam/foo`
DEBUG Adding root workspace member: `/Users/stewartadam/foo`
DEBUG Adding discovered workspace member: `/Users/stewartadam/foo/a`
DEBUG Adding discovered workspace member: `/Users/stewartadam/foo/a/b`
DEBUG Acquired lock for `/Users/stewartadam/foo`
DEBUG Reading Python requests from version file at `/Users/stewartadam/foo/.python-version`
DEBUG Using Python request `3.12` from version file at `/Users/stewartadam/foo/.python-version`
DEBUG Checking for Python environment at: `/Users/stewartadam/foo/.venv`
DEBUG The project environment's Python version satisfies the request: `Python 3.12`
DEBUG Released lock at `/var/folders/1m/xxn68ngs7bb6h6x6bt442tth0000gn/T/uv-d4d39f85f2047a7c.lock`
DEBUG Acquired lock for `/Users/stewartadam/foo/.venv`
DEBUG Using request timeout of 30s
DEBUG Found static `pyproject.toml` for: a @ file:///Users/stewartadam/foo/a
DEBUG Found workspace root: `/Users/stewartadam/foo`
DEBUG Found static `pyproject.toml` for: b @ file:///Users/stewartadam/foo/a/b
DEBUG Project is contained in non-workspace project: `/Users/stewartadam/foo/a`
DEBUG No workspace root found, using project root
DEBUG Found static `pyproject.toml` for: foo @ file:///Users/stewartadam/foo
DEBUG Found workspace root: `/Users/stewartadam/foo`
DEBUG Existing `uv.lock` satisfies workspace requirements
Resolved 3 packages in 5ms
DEBUG Using request timeout of 30s
Audited in 0.00ms
DEBUG Released lock at `/Users/stewartadam/foo/.venv/.lock`
DEBUG Released lock at `/Users/stewartadam/.cache/uv/.lock`
```

`uv sync -v --all-packages` from within the `foo/a/b` folder:
```
DEBUG uv 0.9.7 (Homebrew 2025-10-30)
DEBUG Acquired shared lock for `/Users/stewartadam/.cache/uv`
DEBUG Found project root: `/Users/stewartadam/foo/a/b`
DEBUG Project is contained in non-workspace project: `/Users/stewartadam/foo/a`
DEBUG No workspace root found, using project root
DEBUG Acquired lock for `/Users/stewartadam/foo/a/b`
DEBUG No Python version file found in workspace: /Users/stewartadam/foo/a/b
DEBUG Using Python request `>=3.12` from `requires-python` metadata
DEBUG Checking for Python environment at: `.venv`
DEBUG The project environment's Python version satisfies the request: `Python >=3.12`
DEBUG Released lock at `/var/folders/1m/xxn68ngs7bb6h6x6bt442tth0000gn/T/uv-38125e2740c2addc.lock`
DEBUG Acquired lock for `.venv`
DEBUG Using request timeout of 30s
DEBUG Found static `pyproject.toml` for: b @ file:///Users/stewartadam/foo/a/b
DEBUG Project is contained in non-workspace project: `/Users/stewartadam/foo/a`
DEBUG No workspace root found, using project root
DEBUG Existing `uv.lock` satisfies workspace requirements
Resolved 1 package in 3ms
DEBUG Using request timeout of 30s
Audited in 0.00ms
DEBUG Released lock at `/Users/stewartadam/foo/a/b/.venv/.lock`
DEBUG Released lock at `/Users/stewartadam/.cache/uv/.lock`
```

### Platform

MacOS Tahoe

### Version

uv 0.9.7 (Homebrew 2025-10-30)

### Python version

Python 3.12.12

Reproduce the nested workspace behavior with the documented uv init and uv sync commands, especially uv sync -v --all-packages from foo/a and foo/a/b. Trace the workspace-root discovery shown in the logs and verify that syncing from a/b uses foo/.venv and foo/uv.lock rather than creating local files. Done means nested members consistently resolve to the outer workspace.

Written by the indexing model from the issue text.