Add black-box e2e test for start_agent EventHandler action

[kowser-orkes]

Pull Request type

• Feature
• Bugfix
• Refactoring (no functional changes, no api changes)
• Build related changes
• WHOSUSING.md
• Other (please describe):

NOTE: Please remember to run ./gradlew spotlessApply to fix any format violations.

Changes in this PR

• black-box e2e test suite for the start_agent EventHandler action, against a real running server
• drives the real trigger → queue → EventHandler → agent-config pipeline, no mocks, no in-process shortcuts
• covers 3 outcomes off one shared trigger: bogus-model failure, blocked/terminated tool approval, live-LLM completion

                    REST startWorkflow
                            |
                            v
                 +----------------------+
                 | trigger WorkflowDef  |
                 | EVENT task           |
                 | sink: conductor      |
                 +----------------------+
                            |
                            v
                 +----------------------+
                 | conductor queue      |
                 | (QueueDAO, no infra) |
                 +----------------------+
                            |
                            v
                 +----------------------+
                 | shared EventHandler  |
                 | action: start_agent  |
                 +----------------------+
                            |
                            v
                 +-----------------------------+
                 | agent (real agent-config     |
                 | pipeline via AgentClient)    |
                 +-----------------------------+
                    /          |           \
                   v           v            v
              bogus model  prefillTools   live LLM
              -> FAILED    blocked ->     -> COMPLETED
                            TERMINATED
                    \          |           /
                     v         v          v
                 idempotencyKey -> correlationId
                            |
                            v
                 WorkflowClient.getWorkflows(...)

Issue #CCOR-13315

Depends on

• a conductor-client release containing start_agent — e2e/build.gradle still pins 6.0.0.rc3, does not compile without it or a local workaround

Alternatives considered

• hand-rolled WorkflowDef-based agent, in-process, no real queue — already covered by a separate existing in-process test
• LocalStack SQS-backed trigger queue — unnecessary, EVENT task's sink: conductor already routes through the server's own built-in QueueDAO queue
• unregistered-agent failure path — not black-box-observable, already covered by the existing in-process test