Tests: fail informatively instead of panicking the whole package

[distronode-com]

One failed request takes the whole handler package down

id := created["id"].(string) on a body that has no id panics, and a panic aborts the
entire test binary. So a single bad request reports a nil-interface stack trace and no
other handler test result at all, with the status and error message nowhere on screen.

Measured, not inferred. With a create that really fails (a loopback webhook URL, rejected
by the SSRF guard) plus one unrelated passing test in the same package:

Before

--- FAIL: TestZZOldPattern
panic: interface conversion: interface {} is nil, not string [recovered, repanicked]
FAIL	github.com/calnode/calnode/internal/handler

The unrelated test never reports.

After

zz_panic_demo_test.go:32: create webhook: status = 400; want 201 —
  {"error":"webhook URL must not resolve to a private or loopback address:
   \"127.0.0.1\" resolved to a private or loopback address"}
--- FAIL: TestZZNewPattern
--- PASS: TestZZSomeOtherUnrelatedTest

The diagnosis is on the first line and every other test still reports. (Those two demo
tests were temporary and are not in the diff. That 400 is a real one — it cost real
diagnosis time hidden behind the panic.)

The change

Four helpers beside the existing ones in api_test.go, each taking a what that names the
call so a test making several requests still says which one failed:

mustStatus(t, rec, want, what)                  // status + body on failure
mustJSON(t, rec, want, what)   map[string]any   // status first, then decode
mustCreated(t, rec, what)      map[string]any   // mustJSON for 201
mustString(t, body, key, what) string           // two-value form, or fail with the body

Of the 32 ].(string) sites in internal/handler/*_test.go:

• 10 used the single-value form and could panic. 6 of those (invites) decoded the body
  with no status check at all; 2 checked the status without printing the body. All 10 now
  assert the status first and read fields through mustString.
• 18 already used the two-value form but silently yielded "" for a missing field,
  deferring the failure to something downstream that could not explain it — a 404 on an
  empty id. Those use mustString too.
• 4 are deliberately left alone, because there the two-value form is the assertion:
  test_email_test.go:49, public_eventtype_test.go:64, claim_test.go:81, and the
  secret-length check in webhook_test.go.

No test's assertions changed. Where a status check was missing it was added, which only
makes an already-implicit dependency explicit. No non-test file is touched.

Testing

go test ./... green (26 packages); gofmt -l . empty; go vet ./... clean. Every test
that passed before passes now.

[pullfrog]

✅ No new issues found.

Reviewed changes in this run: full PR diff (test-only helpers + call-site migrations; 1 commit on test/fail-instead-of-panic).

• Informative response helpers — mustStatus / mustJSON / mustCreated / mustString in api_test.go fail the single test with status + body instead of panicking the package or yielding empty ids that 404 later.
• Call-site migration — panic-prone and silent two-value .(string) extracts in availability, invites, MCP OAuth, overrides, webhooks, and a few booking/rule paths now go through those helpers; status is checked before field reads where it was missing.
• Intentionally untouched sites — remaining two-value asserts that are the assertion (and out-of-scope leftovers like calendar_test.go) match the PR description; no production code changed.

go test ./internal/handler/ passed on this head.

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