fix: decouple challenge cleanup from canceled context

[ousamabenyounes]

Run wrapped ACME solver cleanup with a context that preserves values but ignores issuance cancellation. Add a regression test covering cleanup after the obtaining context is canceled.

Test verification (RED → GREEN)

With the fix reverted, the new test fails (RED):

--- FAIL: TestSolverWrapperCleanUpUsesUncanceledContext (0.00s)
    solvers_test.go:42: cleanup context is canceled: context canceled
FAIL
FAIL	github.com/caddyserver/certmagic	0.005s
FAIL

With the fix applied, the test passes (GREEN):

ok  	github.com/caddyserver/certmagic	0.006s

Full local suite

Command: go test -v -short -race ./...

=== RUN   Test_challengeKey/ok/http-ipv4
=== RUN   Test_challengeKey/ok/http-ipv6
=== RUN   Test_challengeKey/ok/tls-ipv4
=== RUN   Test_challengeKey/ok/tls-ipv6
=== RUN   Test_challengeKey/fail/tls-ipv4
=== RUN   Test_challengeKey/fail/tls-ipv6
--- PASS: Test_challengeKey (0.00s)
    --- PASS: Test_challengeKey/ok/dns-dns (0.00s)
    --- PASS: Test_challengeKey/ok/http-dns (0.00s)
    --- PASS: Test_challengeKey/ok/tls-dns (0.00s)
    --- PASS: Test_challengeKey/ok/http-ipv4 (0.00s)
    --- PASS: Test_challengeKey/ok/http-ipv6 (0.00s)
    --- PASS: Test_challengeKey/ok/tls-ipv4 (0.00s)
    --- PASS: Test_challengeKey/ok/tls-ipv6 (0.00s)
    --- PASS: Test_challengeKey/fail/tls-ipv4 (0.00s)
    --- PASS: Test_challengeKey/fail/tls-ipv6 (0.00s)
=== RUN   TestDNSManagerCleanUpRecordPreservesProviderData
--- PASS: TestDNSManagerCleanUpRecordPreservesProviderData (0.00s)
=== RUN   TestGetACMEChallenge_IPv6Brackets
--- PASS: TestGetACMEChallenge_IPv6Brackets (0.00s)
=== RUN   TestTryListen
--- PASS: TestTryListen (0.10s)
=== RUN   TestPrefixAndKeyBuilders
--- PASS: TestPrefixAndKeyBuilders (0.01s)
=== RUN   TestFileStorageStoreLoad
--- PASS: TestFileStorageStoreLoad (0.00s)
=== RUN   TestFileStorageStoreLoadRace
--- PASS: TestFileStorageStoreLoadRace (0.04s)
=== RUN   TestFileStorageWriteLock
--- PASS: TestFileStorageWriteLock (0.00s)
PASS
ok  	github.com/caddyserver/certmagic	1.528s
=== RUN   TestFile
--- PASS: TestFile (0.00s)
=== RUN   TestConcurrentWrites
--- PASS: TestConcurrentWrites (0.00s)
PASS
ok  	github.com/caddyserver/certmagic/internal/atomicfile	(cached)
?   	github.com/caddyserver/certmagic/internal/filedescriptor	[no test files]
?   	github.com/caddyserver/certmagic/internal/testutil	[no test files]

Fix #313

[mholt]

Thanks! This is probably a good change.