fix(api): raise domain exceptions where the caller needs the message

[ExtraToast]

What

Follow-up to JorisJonkers-dev/kotlin-spring-commons#34, which stops the shared advice returning IllegalStateException's message. error(…) throws exactly that, so every error(…) in this service now produces a generic 409.

For most of them that is correct — "No handler registered for X", "not visible immediately after create", "empty response from gateway", "workspace missing for session " are internal invariants and should stay withheld. Eight are not, and move onto the exception types this repo already uses elsewhere.

Conflicts the caller caused and needs to read → DomainException with a code, so the message survives as a 400:

handler	code
CreateProjectCommandHandler	SLUG_ALREADY_IN_USE
CreateRepositoryCommandHandler	REPOSITORY_ALREADY_REGISTERED

Lookups of an entity the caller named → NotFoundException, which is both the right status and a detail worth reading. These were 409s:

• LinkRepositoryToProjectCommandHandler — project, repository
• AddGithubLinkCommandHandler — project
• OpenPullRequestCommandHandler — workspace
• AgentSessionController — session (×2), workspace

Deliberately unchanged

SendUserInputCommandHandler's generation conflict and AgentSessionController.restartConflict. agents-ui reads a 409 from the restart path to drive its reattach flow (workspaces.ts, err.status === 409). That contract is status-only — it never reads the detail text — so the generic detail does not affect it, and neither status changes here.

Status changes

Six paths move 409 → 404. Nothing in api/src/test, api/src/integrationTest or the UIs asserts a 409 on any of them; the only 409 assertions are on the restart-conflict path above, which is an explicit ResponseEntity, not an exception.

Tests

Updated the five handler tests that asserted IllegalStateException to assert the new type and that the message still carries the slug / URL / id — the point of the change is that these messages survive, so the tests check the message, not just the type. Added an AgentSessionController test asserting the unknown-session path is a 404 whose detail still names what was missing.

Verification

Not built locally — ./gradlew here resolves dev.jorisjonkers:kotlin-commons-*:0.7.1 from GitHub Packages and this workstation shell has no gpr.user / gpr.token, so compilation fails at dependency resolution before reaching any source. Relying on CI for compile, test, ktlint and detekt; happy to re-run locally if someone points me at credentials.