test: remove tests that do not pin user-visible behavior (#16539)

* test: remove tests that do not pin user-visible behavior

Apply the AGENTS.md test rules to the existing Rust and Flutter suites.
A test is kept when it is the only coverage of user- or API-visible
behavior, the regression test of a reported or fixed bug, a security
boundary (one test per rejection class), or a check against an external
reference. Tests that assert private state or the order of internal
steps, pin review-round corner cases, or repeat what a kept test already
covers are removed.

Rust: 679 -> 298 tests. Dart: 118 -> 45 tests.

Test-only helpers left without callers go with them
(applyLastOnlineStateForTesting, terminalKeyboardRowWidth,
debugBufferedInputCount, tokio's test-util dev feature), and cfg gates
and visibility that existed only for the deleted tests are narrowed.
Runtime behavior is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WS1HTjp6UQuG43KEcQPq1R

* test: keep tests that pin security and user-visible behavior

Restore six tests that review showed are the only coverage of their
behavior:

- update metadata rejects an artifact file name with path components;
- ICE candidates from another session are dropped;
- KCP delivers every multi-segment frame queued before a close;
- set_files rejects an empty name in a multi-file transfer;
- a lopsided elongated cursor keeps its centre hotspot (#16242);
- toggling Show hidden during a pending same-path read issues a new read.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WS1HTjp6UQuG43KEcQPq1R

* test: keep offline update and key-exchange downgrade rejections

Restore three tests that are the only coverage of their rejection class:

- offline update metadata rejects a signed artifact with an http or
  credential-bearing URL;
- offline verification rejects a renamed local file and a version that
  does not match the release;
- a key-exchange pick lowered in transit leaves both sides unable to
  decrypt, so a v0 fallback after a failed v1 decrypt cannot pass
  unnoticed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WS1HTjp6UQuG43KEcQPq1R

* test: keep the decomposed-grapheme and reconnect-cancel tests

Each catches a plausible wrong change that passes every kept test:
mapping the first scalar under a locked Ctrl turns é into \x05,
and a reconnect that does not fail pending reads leaves them hanging
until the 30 s timeout.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WS1HTjp6UQuG43KEcQPq1R

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/core_main.rs

@@ -900,59 +900,6 @@ fn parse_silent_install_args(args: &[String]) -> (Option<bool>, bool) {
     (printer_override, debug)
 }
 
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    fn args(values: &[&str]) -> Vec<String> {
-        values.iter().map(|value| value.to_string()).collect()
-    }
-
-    #[cfg(any(target_os = "linux", target_os = "macos"))]
-    #[test]
-    fn user_main_ipc_scope_cli_command_matches_management_commands_only() {
-        for command in [
-            "--password",
-            "--set-unlock-pin",
-            "--get-id",
-            "--set-id",
-            "--config",
-            "--option",
-            "--assign",
-            "--deploy",
-        ] {
-            assert!(is_user_main_ipc_scope_cli_command(&args(&[command])));
-        }
-
-        for command in [
-            "--service",
-            "--server",
-            "--tray",
-            "--cm",
-            "--check-hwcodec-config",
-            "--connect",
-        ] {
-            assert!(!is_user_main_ipc_scope_cli_command(&args(&[command])));
-        }
-    }
-
-    #[cfg(target_os = "macos")]
-    #[test]
-    fn macos_manual_update_result_preserves_failure() {
-        let failure =
-            finish_macos_update(Err(hbb_common::anyhow::anyhow!("administrator canceled")))
-                .expect("failure action");
-
-        match failure {
-            CoreMainAction::ExitFailure(err) => {
-                assert_eq!("administrator canceled", err.to_string());
-            }
-            CoreMainAction::StartUi(_) => panic!("update failure started the UI"),
-        }
-        assert!(finish_macos_update(Ok(())).is_none());
-    }
-}
-
 /// Check if the executable is a Quick Support version.
 /// Note: This function must be kept in sync with `libs/portable/src/main.rs`.
 #[cfg(windows)]