github.bot_login and trigger.mention_login were read raw in the
require_mention precedence chain, so a whitespace-only value (e.g. " ")
never fell through to the working fallback the chain documents -
Python truthiness lets " " win an `or` chain over agent.name or the
operator default. The third link (channels.github.default_mention_login)
was already correctly normalized and pinned by
test_operator_default_blank_string_treated_as_none; this closes the gap
for the other two by normalizing GitHubTriggerConfig.mention_login and
GitHubAgentConfig.bot_login once, at the config layer, via a
field_validator mirroring the pattern already used elsewhere in the
config package.
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