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/lang.rs

@@ -336,16 +336,6 @@ mod test {
         );
     }
 
-    #[test]
-    fn test_resolve_lang_forces_english_for_saved_cjk_when_target_disables_cjk() {
-        use super::resolve_lang as f;
-
-        assert_eq!(f("zh-cn", "en-US", true), "en");
-        assert_eq!(f("zh-tw", "en-US", true), "en");
-        assert_eq!(f("ja", "en-US", true), "en");
-        assert_eq!(f("ko", "en-US", true), "en");
-    }
-
     #[test]
     fn test_resolve_lang_forces_english_for_cjk_locale_when_target_disables_cjk() {
         use super::resolve_lang as f;
@@ -375,12 +365,4 @@ mod test {
         assert_eq!(f("", "pt-AO", false), "pt-pt");
         assert_eq!(f("", "pt-MZ", false), "pt-pt");
     }
-
-    #[test]
-    fn test_resolve_lang_saved_lang_takes_precedence_over_locale() {
-        use super::resolve_lang as f;
-
-        assert_eq!(f("pt-br", "pt-PT", false), "pt-br");
-        assert_eq!(f("pt-pt", "pt-BR", false), "pt-pt");
-    }
 }