env2bool matches its truthy pattern as a substring, so unrelated values read as true

Bug Report

dvc.utils.env2bool decides truthiness with an unanchored re.search:

def env2bool(var, undefined=False):
    var = os.getenv(var, None)
    if var is None:
        return undefined
    return bool(re.search("1|y|yes|true", var, flags=re.IGNORECASE))

Because search matches anywhere in the string, any value that merely contains 1, y, yes or true is treated as true — including values that plainly aren't boolean:

'1'          -> True     # intended
'yes'        -> True     # intended
'true'       -> True     # intended
'0'          -> False    # intended
'no'         -> False    # intended
'false'      -> False    # intended
'off'        -> False    # intended

'my_path'    -> True     # contains "y"
'anything'   -> True     # contains "y"
'only'       -> True     # contains "y"
'v1.0'       -> True     # contains "1"

So a variable set to a path, a branch name, a version string, or anything else containing a y or a 1 silently reads as enabled.

Impact

env2bool is used for user-facing environment variables, so the input really is arbitrary user text:

• DVC_EXP_AUTO_PUSH — dvc/repo/experiments/executor/base.py
• DVC_SQLALCHEMY_ECHO — dvc/database.py
• DVC_IGNORE_ISATTY — dvc/progress.py
• DVC_TEST — dvc/analytics.py, dvc/repo/experiments/push.py, dvc/updater.py

The most likely way to hit it is a value meant to be negative that happens to contain a matching letter (deny, nay, not_yet all read as true), or someone putting a non-boolean value in one of these by mistake and getting the feature switched on rather than an error or a sensible default.

It's also asymmetric in a confusing way: no is correctly false, but nay is true.

Expected

Only a recognised affirmative value should be true — i.e. match the whole string, not a substring. Something like

return var.strip().lower() in {"1", "y", "yes", "true"}

or re.fullmatch(r"1|y|yes|true", var.strip(), flags=re.IGNORECASE) if you'd rather keep the regex. Note that with fullmatch the yes alternative is still needed (it isn't reachable via y any more), which is presumably why search looked sufficient originally.

Reproduce

import os
from dvc.utils import env2bool

os.environ["DVC_EXP_AUTO_PUSH"] = "my_branch"
print(env2bool("DVC_EXP_AUTO_PUSH"))   # True

Environment

DVC main (56e5982), Python 3.11.9, Windows.

Happy to send a PR with the fullmatch/set-membership version and a test if you agree with the direction — flagging first since tightening this is technically a behaviour change for anyone currently relying on a loose value.

Start at dvc.utils.env2bool and reproduce the issue with the DVC_EXP_AUTO_PUSH environment variable and a value such as my_branch. Check the existing utility test coverage, then ensure only whole, recognised affirmative values return true while unrelated values do not. Add a regression test for the substring case and run the relevant utility tests.