Add context-aware connection tests, spec events, and workbench env config

[flarco]

Problem

sling conns test can only be configured through process-wide SLING_TEST_* env vars, and spec test events go to a package-global channel (api.SpecEventChn). Two tests running at once share one channel, a test cannot be cancelled, and a server (the workbench) has no way to pass per-run options, receive structured events, or stop a run. There is also no way to test a connection against an edited spec without saving it, and no place in env.yaml to configure sling serve workbench.

Solution

ELI5: instead of settings shouted through global env vars and one global megaphone, every test run now carries its own settings and its own private callback, handed over through the context. Runs can overlap, each can be stopped, and each reports its own progress. A run can also point at a draft spec just for that run, and env.yaml can hold workbench settings.

Details:

• api.SpecEvent and typed event constants (endpoint-start, request-complete, records, endpoint-done, error) replace the map[string]any global channel. WithSpecEventHandler/fireSpecEvent carry the handler on the request context, so events are per-run and cancellation propagates. Legacy JSON fields (req_id, timestamp, iter_id, iter_sequence, size_bytes, record_count) are kept for released spec inspectors.
• request-complete events now include request_index, state_before and state_after (iteration state read under iter.context), plus each request's records.
• Connection.TestWithOptions(ctx, TestOptions) and ConnEntries.TestWithOptions(ctx, name, opts) add endpoints, limit, max requests, context, spec file, trace and OnEvent. Test() keeps its signature and builds its options from SLING_TEST_*, so the CLI is unchanged.
• TestOptions.SpecFile overlays a spec on a copy of the connection entry for one run; the original entries stay untouched.
• A cancelled test returns context.Canceled instead of success, and stops before the next endpoint.
• env.yaml gains a managed workbench: block (WorkbenchConfig: host, port, token, projects_root, shell, worker_idle, path_extra, env); command-line flags win over the file.
• The goreleaser mac formula installs a launchd service running sling serve workbench --no-browser.

Testing

• core/dbio/connection/connection_test.go: event sequence and payloads (request index, state before/after, legacy wire fields), error event, env-driven test, spec-file overlay, and cancellation via context.
• tests/suite.cli.yaml cases 620 and 621: workbench --help output, plus a smoke test that starts the server on a free port, checks /healthz returns ok, and stops it.