Move the inline tests and test-only setters of session.rs (app-logic) into separate files #837

Goal

crates/app-logic/src/session.rs mixes the session type, about 20 test-only setters and a large inline test module. Move the tests and the test-only setters into their own files so the production session code reads on its own. No behaviour changes.

Context

• crates/app-logic/src/session.rs: 2,678 lines.
• Inline test module #[cfg(test)] mod tests from :1924 to the end.
• Test-only setters inside impl Session (:1280-1431), each gated with #[cfg(any(test, feature = "test-support"))] (Spec 0086 A3.3 / ADR 0080 decision 4, Spec 0085 A3.3, Spec 0084 A5): set_*_for_tests, set_sftp_for_tests, … They are reachable from other crates' tests through the test-support feature and must never be callable from production builds (cargo build --workspace builds without test-support and must reject such a call).
• File naming convention for new test files: <module>/tests.rs or <module>/tests_<topic>.rs; no new *_tests.rs.

Requirements

• Move mod tests to crates/app-logic/src/session/tests.rs (#[cfg(test)] mod tests;).
• Move all #[cfg(any(test, feature = "test-support"))] setters of impl Session into a second impl Session block in crates/app-logic/src/session/test_setters.rs, declared from session.rs as #[cfg(any(test, feature = "test-support"))] mod test_setters;. Keep the explanatory comment block (:1280-1288) at the top of that file and keep the per-function #[cfg(any(test, feature = "test-support"))] attributes or replace them by the module-level gate (either is fine; the effective gate must stay exactly any(test, feature = "test-support")).
• Visibility of each setter stays the same (pub stays pub, pub(crate) stays pub(crate)); callers in other crates compile unchanged.
• Production methods between the setters (e.g. take_queued_user_messages, request_auto_continue_stop) stay in session.rs.
• Each new file starts with a //! line.

Acceptance Criteria

• session.rs contains no inline #[cfg(test)] mod … { block and no *_for_tests method; it has fewer than 2,000 lines.
• cargo build --workspace (without test-support) still fails to compile a production call to a moved setter: the PR description shows the check (temporarily add a call in a production function, run cargo build -p app-logic, see the error, revert).
• cargo test -p app-logic --lib -- --list 2>/dev/null | grep -c ': test$' is identical before and after, the set of test names unchanged; the PR description states both numbers.
• All crates that use the setters (grep -rn '_for_tests' crates/ apps/) compile and their tests pass.
• No new file matching *_tests.rs.
• cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace, cargo build --workspace pass.

Technical Notes

• set_filter_engine_for_tests, set_ai_provider_for_tests and set_sudo_password_for_tests touch security-relevant session state (see .claude/skills/smart-ssh-coder/references/security-invariants.md). The risk is only in the gate: the moved code must be exactly as unreachable from production as before.
• Splitting SessionParts into sub-structures is separate work that builds on this issue.

Spec impact: none