fix: support nested routers in FastAPI 0.140

[LilScottyPippen]

Fix default_limits not applied with FastAPI >= 0.140

FastAPI changed route internals and introduced _IncludedRouter.
SlowAPI middleware couldn't resolve endpoint handlers anymore,
causing default_limits and application_limits to silently skip.

This patch adds recursive route resolution for new router structures.

[Mukller]

Triage note for the cluster of PRs fixing included-router rate limiting (#285, #286, #282): I verified all three functionally on FastAPI 0.141.1 with a uniform probe (default_limits only, SlowAPIMiddleware, decorator-free endpoint — the path where only _find_route_handler can enforce):

app.include_router(sub)            -> second request status
app.include_router(sub, prefix="/sub")

master : 200,200 BYPASSED   | 200,200 BYPASSED     (bug confirmed)
#285   : 200,429 ENFORCED   | 200,429 ENFORCED
#286   : 200,429 ENFORCED   | 200,429 ENFORCED   <- this PR
#282   : 200,429 ENFORCED   | 200,200 BYPASSED    <- prefixed case still leaks

Findings specific to this PR (as posted on #285):

• Your _effective_candidates + .routes recursion works against real 0.141 internals (_IncludedRouter exposes both).
• One semantic difference vs the other two candidates worth knowing about: #286 returns on the first FULL match, while #285 keeps the original last-match-wins scan (handler = ...; continue) — see note on #285; on apps where several routes could match the same scope this may differ from pre-0.137 slowapi behavior. On apps where several routes could match the same scope, behavior may differ from pre-0.137 slowapi — might be worth an explicit decision/test.
• Your suite passes locally for me (61 passed, incl. the new default-limits integration test).

Version coverage question for maintainers: #285 keys off effective_route_contexts (≥0.137 lazy inclusion), #286 keys off _effective_candidates (the _IncludedRouter attr present in ≥0.140 builds I inspected), and #282 tries .routes/original_router.routes. All three attributes coexist on 0.141.1 — so the practical choice is about which older FastAPI lines you want to keep supporting, since the attribute landscape changed between 0.137–0.140.