Robust environment variable handling

When implementing features that read or set configuration from environment variables (e.g. loading .env files, FLASK_ENV, FLASK_APP, FLASK_DEBUG), be defensive and explicit about parsing, scope, and documentation:

1. Handle string/bytes encoding consistently across Python versions. Don’t compare native strings to bytes when reading files in binary mode, and don’t assume text decoding works the same on Py2/Py3.
2. Validate input lines/values and bail out (or raise a clear error) on malformed or unsupported formats (e.g. multiline or quoted values) instead of silently truncating or misparsing them:

for line in f:
    line = line.strip()
    if not line or line.startswith(b'#'):
        continue
    if b'=' not in line:
        raise ValueError('Invalid line in .env file: %r' % line)
    key, _, value = line.partition(b'=')
    _set_env(key, value)

1. Respect intended scope/boundaries when searching for or loading configuration files. If a search is meant to stop at a project root (top_dir), enforce that boundary explicitly rather than relying on directory traversal alone:

def find_dotenv(top_dir=None):
    here = os.getcwd()
    while 1:
        path = os.path.join(here, '.env')
        if os.path.isfile(path):
            return path
        if top_dir is not None and here == top_dir:
            break
        there = os.path.dirname(here)
        if there == here:
            break
        here = there

1. Clearly document how and when configuration values take effect (e.g. that DEBUG/FLASK_ENV should only be changed via environment variables before the app starts, not mutated directly on the app object), to avoid confusion about stale or recalculated config values. Add tests covering edge cases like malformed .env lines, out-of-scope .env files, and environment variables set after app configuration is computed.</description>