test(marketplace): pin the agent detail page's load path

[philmerrell]

Adds the spec AgentDetailPage never had. Follow-up to #740 — that PR fixed the bug, this one
makes it impossible to reintroduce silently.

Why this is worth a PR of its own

The phase 3 bug (#740) shipped from 66f38346 until yesterday, and the reason nobody caught it
is entirely the missing spec: the page rendered a plausible "Failed to load this agent." banner
with no HTTP request behind it — no console error, no failed request, nothing to grep for.
/agents/:id had never successfully loaded in a browser.

The one thing that makes this test worth having

The component is routed through RouterTestingHarness with
provideRouter(routes, withComponentInputBinding()) rather than constructed directly.

That is load-bearing, not ceremony. id is an input.required<string>() that the router sets
after construction — which is the entire bug. A spec that hands id in at construction time
(TestBed.createComponent with inputs pre-supplied) passes against the broken code and proves
nothing. There's a comment saying so at the top of the file, because this is exactly the kind of
setup a future reader would try to simplify away.

The two assertions that would have caught it are deliberately not about the rendered agent:

• AgentApiService.getAgent is actually called, with the bound id
• the error banner is absent on the happy path

A zero-request page is what the bug produced, so those are the two facts that matter.

Verified both directions

Component state	Result
constructor() (the bug, pre-#740)	5/5 fail — getAgent "called 1 times, but got 0 times"; error() = 'Failed to load this agent.'
ngOnInit() (#740, now on develop)	5/5 pass

Run on this branch: 5 passed. Full src/app/agents/** suite: 8 files, 77 tests, green.

npx ng test --include src/app/agents/detail/agent-detail.page.spec.ts --watch=false

Coverage

Five tests over the load path: request-issued-with-bound-id; no error banner on success;
runnability fetched separately for the same id; a failing-read negative control (without it,
"banner absent" could pass on a page incapable of rendering one); and the advisory-runnability
path, where a runnability failure must not error the whole page (D6).

Services are stubbed via DI tokens rather than vi.mock, per the repo convention on cross-spec
mock pollution. No production code is touched — one new file, 155 lines.

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