fix(NATSRS-001-2): CU-86akbh48z service/mod.rs Service::add returns Box<dyn Error> style ad-hoc io::Error instead of a typed *ErrorKind

[flamingo]

Closes findings from rule NATSRS-001-2 — service/mod.rs Service::add returns Box style ad-hoc io::Error instead of a typed *ErrorKind.

Draft — this is a starting point, not a finished change. The fix required judgment, so read it before trusting it.

#		Finding	Location
1	🔴	service/mod.rs Service::add returns Box style ad-hoc io::Error instead of a typed *ErrorKind	async-nats/src/service/mod.rs:330
2	🟠	service task loop uses unwrap() on ping.reply / info_request.reply / stats_request.reply, panicking the background task on missing reply subject	async-nats/src/service/mod.rs:390
3	🟠	info_request.reply also unwrapped without guard, same panic risk as ping handler	async-nats/src/service/mod.rs:410

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Run: https://product-hub.flamingo.so/admin/code-review
Run id: 06b1d724-91f9-455e-9eef-4cda91351dba

Merging this PR is recorded as acceptance of the rule that produced it;
closing it unmerged is recorded as rejection. Both feed rule health, so
closing a wrong suggestion is useful rather than merely tidy.

ClickUp task: CU-86akbh48z NATS async-nats and CI review findings (10 PRs)