app: use explicit ValueError instead of assert in projects_dir_validate (fixes #5403)

[adityachilka1]

Fixes #5403.

The bug

platformio/app.py:

def projects_dir_validate(projects_dir):
    assert os.path.isdir(projects_dir)
    return os.path.abspath(projects_dir)

Python drops assert statements under -O (and when the PYTHONOPTIMIZE
environment variable is set). On production images and distro packages that run
Python in optimised mode, any string - even a fake path - gets through the
validator untouched. The error then surfaces downstream (e.g. when the projects
directory is later globbed), where it is harder to diagnose as a config issue.

The fix

Replace the assert with an explicit raise ValueError. One-line diff:

 def projects_dir_validate(projects_dir):
-    assert os.path.isdir(projects_dir)
+    if not os.path.isdir(projects_dir):
+        raise ValueError("Not a directory: %s" % projects_dir)
     return os.path.abspath(projects_dir)

Why ValueError

Caller (sanitize_setting, platformio/app.py:168) wraps any exception from
validators into exception.InvalidSettingValue via raise ... from exc. So
the user-facing error path is identical. ValueError is idiomatic for
"argument has the correct type but wrong value".

Tests

Adds tests/misc/test_app.py with three cases:

Verification (local)

python -O -c "from platformio import app; app.projects_dir_validate('/nope')"
# Traceback (most recent call last):
# ...
# ValueError: Not a directory: /nope

Also verified that the pre-fix code (replicated in a sandbox) silently returned
an invalid path under -O.

No behavioural regression

• Normal (non-optimised) runs: previously assert raised AssertionError
  (a subclass of Exception) which sanitize_setting caught. Now raises
  ValueError which it still catches. No user-facing difference.
• Optimised (-O) runs: was a silent pass-through. Now correctly raises.

Related

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