fix(runners): scope schema-validation crashes to the failing case

[Saladerl]

Contain per-case adapter failures in the flow and server-verify runners

AdapterClient.call() validates every adapter response against
adapter-response.schema.json and the operation's successRef — and it
raises (ValueError) on a violation rather than returning an error
response:

# harness.py
validate_value(response, "adapter-response.schema.json", ...)
if response.get("ok") is True:
    validate_value(response["value"], {"$ref": success_ref}, ...)  # raises

The vector runner wraps every call() per check
(run_operation_timed / run_adapter), so a schema-invalid response there
fails just that one check. The flow runner and the server-verify runner do
not: their only exception handler is the per-adapter one in main().

Consequence: if an adapter returns one schema-invalid value — say its
challenge.parse output for flow case 5 drops a required field — the raise
propagates out of the case loop and the whole adapter collapses into a single
adapter-run failure. The ~30 other flow cases that ran (or would have run)
report nothing, and the failure message doesn't say which case or operation
was at fault. The result is technically red, but useless for diagnosis and
wrong about scope.

Changes

• flow_runner.py: new run_flow_case_safely() wrapper used by
  run_adapter_flows(). A case-level exception now yields a normal per-case
  result with error_type: "case_error: <message>", which the golden
  comparison reports as that case failing — all other cases still run and
  report individually.
• server_verify_runner.py: the per-case client.call() is wrapped the same
  way, appending a failing RunResult for the case and continuing the loop.
• flow_runner.py shutdown: server.kill() in the finally block is now
  followed by server.wait() so an unresponsive compliance server is reaped
  instead of lingering as a zombie for the rest of the CI job.

No behavior change for adapters that respond correctly; the golden comparison
path and result shapes are untouched.

Verification

test_flow_case_isolation.py (new; auto-discovered by ci.yml) checks both
directions: an exception from run_flow_case becomes a scoped per-case
failure carrying the original message, and a successful case result passes
through unchanged. Full script suite: 11 tests, OK.