fix: list subscriptions through v2 and paginate the method that was called

[gastonrey]

Both of these are listed as Known issues in the 0.2.1 changelog. One of them is worse than
recorded, and the example covering it was asserting the bug.

#list always raised

Subscriptions#list used base_path, which is /v1/subscriptions — a route that does not
exist. Verified against the sandbox:

subscriptions.list  ->  NotFoundError 404: Route not found

The changelog described this as "uses the v1 path while #get/#update and search use v2",
which reads like an inconsistency. It is a method that could never return.

base_path stays on v1, because the action sub-paths genuinely live there: POST /v1/subscriptions/{id}/update is what the API accepts, and so are cancel, pause,
activate, reactivate, reinstate and renew. Moving base_path wholesale breaks 21
examples. Only the collection GET moves, onto the same search_path that search already uses.

The existing example stubbed /v1/subscriptions, so it passed whether or not the endpoint
existed — the same failure mode as the update_operation payload corrected in #10: a stub
validating a shape nobody had checked against the API.

#next_page left the endpoint it came from

Collection#next_page re-issued @resource.list regardless of which method produced the page,
so a page from a custom list method paged into the resource's default endpoint. Today
Subscriptions#list_templates is the only such method, and the endpoint it would have jumped to
is the 404 above.

Collection now takes the method that produced it, defaulting to :list, and next_page calls
that. Existing callers are unaffected.

Verification

• 110 examples, up from 105. RuboCop clean across 35 files.
• New collection_spec.rb covers the default path, the custom-method path, the last page, and
  auto_paginate over a custom method.
• Three mutations, three caught: next_page back to the default list (2 failures), list back
  to resource_path (1), and base_path moved to v2 (21 — which is the evidence for leaving it
  alone).
• Live against the sandbox with this branch loaded: list returns 29 subscriptions,
  next_page returns a disjoint page, auto_paginate walks all 29, and list_templates
  answers 0 rows with next_page nil instead of reaching for another endpoint.

A correction

An earlier read of this claimed list_templates(...).next_page raised NoMethodError. It does
not: the sandbox has zero templates, so next_page correctly returned nil and the probe called
.data on it. The pagination bug here is the wrong-endpoint one above, pinned by a spec rather
than by a live reproduction.

Still open, not in this PR

• The write retry budget is a hardcoded MAX_WRITE_RETRIES = 2 while Configuration already
  carries max_retries for the read retry — two budgets, one unreachable.
• push_hyperline_catalog.rb in the monolith still monkey-patches BaseResource#request,
  because it needs a hard timeout, retries on 404 and 409, poison detection, and nonce-carrying
  keys. Until retry policy is configurable here, any signature change in this gem breaks that
  script silently — which is exactly what feat: write safety, error classes, and the Features resource #10 did.