Relay-plane parity: hermes config set / Railway persist YAML booleans
as strings, and _slack_unfurl_kwargs silently dropped them — so
'unfurl_links: "false"' was a no-op on native while working on relay.
Coerce recognized string booleans exactly as _slack_unfurl_hints does;
unrecognized values still drop so junk config keeps Slack's default
instead of accidentally suppressing previews.
Replaces test_send_ignores_non_boolean_unfurl_options (which froze the
dropped-string behavior) with coercion + junk-drop tests.
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