Fix wrong Python executable path on Linux/macOS after creating conda environments

[StellaHuang95]

Part of #1454

Bug

In createNamedCondaEnvironment and createPrefixCondaEnvironment, the Python executable path is computed as:

const bin = os.platform() === 'win32' ? 'python.exe' : 'python';
// Then: path.join(envPath, bin)

On Linux/macOS, this produces /path/to/env/python, but conda installs the Python executable at /path/to/env/bin/python.

Why it's a bug

The environment appears in the VS Code picker and can be selected. However, the internal execInfo.run.executable points to a non-existent path. Any operation that invokes Python — running scripts, linting, getting version info — fails silently. Users see a valid-looking conda environment that they can select, but nothing works after selecting it.

The quickCreateConda function (line 1225) already had the correct path:

os.platform() === 'win32' ? path.join(prefix, 'python.exe') : path.join(prefix, 'bin', 'python')

Repro steps

1. On Linux or macOS, create a conda environment through the VS Code UI (either named or prefix type).
2. The environment appears in the picker — select it.
3. Try running a Python script or checking the Python version.
4. Operations fail because the executable path /path/to/env/python does not exist.

Fix

Changed both createNamedCondaEnvironment (line 1085) and createPrefixCondaEnvironment (line 1169) to use:

const bin = os.platform() === 'win32' ? 'python.exe' : path.join('bin', 'python');

This matches the correct pattern already used in quickCreateConda.

Tests added

• condaUtils.pythonExePath.unit.test.ts: 3 test cases verifying correct executable paths for both platform types and both environment types.