test: replace checkbox assertions with real assertions or DoesNotPerformAssertions

[miaulalala]

Summary

Seven test methods across five files used `assertTrue(true)` or `addToAssertionCount(1)` purely to silence PHPUnit's "no assertions" warning, providing no actual verification of behaviour.

• ClearFrontendCachesTest::testRun — already had mock expectations (assert-once for `clear`, `resetCache`, `createDistributed`); the trailing `assertTrue(true)` was pure noise
• BrokerTest::testNewConversationOptions — `newConversationOptions()` returns an `IConversationOptions` object; now asserts the return type rather than nothing
• SharedQueryBuilderTest (3× validate tests) — each test verifies that `validate()` does not throw for a legitimately valid query; replaced `assertTrue(true)` with `#[DoesNotPerformAssertions]`, which makes the intent explicit without pretending there's an assertion
• PublicShareMiddlewareTest (3× no-op path tests) — same pattern: tests verify no exception is thrown when middleware hits a non-exceptional path; replaced `assertTrue(true)` with `#[DoesNotPerformAssertions]`
• InMemoryFileTest::testDelete — `delete()` is documented as a no-op for in-memory files; replaced the workaround comment + `assertTrue(true)` with `#[DoesNotPerformAssertions]`

Why

Tests that assert `assertTrue(true)` give a false sense of coverage. When a method's behaviour changes in a breaking way, these tests stay green and provide no signal. `#[DoesNotPerformAssertions]` is honest: it explicitly says "this test passes if it doesn't throw", which is at least a truthful contract.

Test plan

• All five modified test classes pass locally with `NOCOVERAGE=1 ./autotest.sh sqlite`
• No new PHPUnit warnings introduced

🤖 Generated with Claude Code

[miaulalala]

/backport to stable34

[susnux]

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