fix(mqtt): mask the broker password in native error lines

[lxsaah]

Found during the stage 12 live run (#290). The native (rumqttc) backend logged the broker URL verbatim whenever its event loop hit a connection error, including any password in the URL's authority:

MQTT event loop error for mqtt://aimdb:nope@localhost:1884: ConnectionRefused(NotAuthorized)

That line repeats every 5 s while the broker refuses the client. The problem predates design 054. It lands on the feature branch rather than main, because stage 11 rewrote this code.

Change (native.rs)

• New redacted(url): masks the password in the URL's authority the way ConnectorUrl's Display already does (mqtt://user:****@host:port). The username, host, port and path stay readable.
• The event loop's log key is redacted(broker_url) instead of broker_url.to_string().
• A second, narrower leak: ConnectorUrl::parse's only error ("missing scheme") quotes its input, so a mistyped broker URL such as user:s3cret@host:1884 came back in the build error Invalid MQTT URL: …, which applications are likely to log. That message now passes through redacted too. A URL without :// is masked the same way.
• The embedded backend wasn't affected: its URL errors are fixed strings, and it never logs the URL.

Tests

• redacted_masks_only_the_password: a password containing @ and :, a URL with a path, a URL without a scheme, and URLs with nothing to mask (no userinfo, username only, not a URL at all).
• an_invalid_url_error_does_not_repeat_the_password: the build error for user:s3cret@localhost:1884 doesn't contain the password. Without the masking, the test fails (… Missing scheme in URL: user:s3cret@localhost:1884/dummy); I checked by removing it.

Verification

• cargo test -p aimdb-mqtt-connector --features std: 48 passed.
• Clippy on std, std,tokio-rustls and std,tokio-native-tls (--all-targets -D warnings) and cargo fmt --all --check: clean.
• CI does not run on PRs into feat/054-connector-boundary.

The changelog entry follows with the rest of 054 in stage 17.

🤖 Generated with Claude Code