Tighten `start_or_cancel`'s startup-RTE match

The `'started' in rte.args[0]` check was too loose: it
matched a child task's OWN `RuntimeError(...started...)`
(not just trio's "child exited without calling
task_status.started()"), and would `TypeError` on a
non-`str` `args[0]`. Guard with `isinstance(..., str)` and
match trio's exact wording so a real child error can't be
demoted to `Cancelled` under cancellation.

Review: PR #464 (copilot-pull-request-reviewer)
https://github.com/goodboy/tractor/pull/464#pullrequestreview-4548203343

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tractor/trionics/_taskc.py

@@ -340,7 +340,16 @@ async def start_or_cancel(
         if (
             rte.args
             and
-            'started' in rte.args[0]
+            isinstance(rte.args[0], str)
+            and
+            # match trio's *exact* "child exited without calling
+            # task_status.started()" wording — a bare `'started'`
+            # substring would also match a child task's OWN
+            # `RuntimeError(...started...)` and (under cancellation)
+            # demote it to a `Cancelled`, losing the real error. The
+            # `isinstance` guard also avoids a `TypeError` when
+            # `rte.args[0]` isn't a `str`.
+            'child exited without calling' in rte.args[0]
         ):
             # re-raises the in-flight `trio.Cancelled` IFF we're
             # under effective cancellation; else a cheap no-op and