UTS: repair broken rest/unit fixtures

[owenpearson]

Fixes the four broken fixtures in uts/rest/unit reported in #528.

BLOCKING fixes (made their tests unsatisfiable for a conforming SDK)

1. presence/rest_presence.md — truncated cipher fixture (RSP5/decode-cipher-channel-7)
The old ciphertext HO4cYSP8LybPYBPZPHQOtuD53yrD3YV3NBoTEYBh4U0= decodes to 32 bytes (16-byte IV + a single AES-CBC block) — one block short of holding any 17-byte-plus PKCS#7-padded plaintext, so it can never decrypt/unpad cleanly. Replaced with the full 48-byte interop ciphertext from ably-common test-resources/crypto-data-128.json (line 41), corrected the comment to {"example":{"json":"Object"}}, and changed the type-only assertion to assert the decrypted value.

Derivation/validation: with the fixture's own key WUP6u0K7MXI5Zeo0VppPwg==, the new ciphertext is 48 bytes (multiple of 16), its first 16 bytes equal the crypto-data-128.json iv, and AES-128-CBC decrypt + PKCS#7 unpad yields {"example":{"json":"Object"}} (pad byte 0x03). Verified locally with cryptography.

2. encoding/message_encoding.md — invalid base64 fixture (RSL6b/unrecognized-encoding-preserved-0)
The old payload encrypted-data-here has 17 alphabet chars (≡1 mod 4) — a residue class no base64 string can occupy, so the base64 decode the test asserts can never succeed. Replaced with ZW5jcnlwdGVkLWRhdGEtaGVyZQ== (base64 of encrypted-data-here) and strengthened the assertion to check the decoded bytes. Added a separate RSL6b failure-branch test (invalid-base64-preserved-1) with a deliberately invalid payload (@@@invalid@@@) asserting the message is delivered with the last successful decoding — data unchanged and encoding == "custom-encryption/base64" — which previously had no coverage in uts/rest/unit.

Validation: ZW5jcnlwdGVkLWRhdGEtaGVyZQ== has length 28 (≡0 mod 4), decodes to encrypted-data-here, and round-trips. @@@invalid@@@ has 7 alphabet chars (≡3 mod 4) and fails strict RFC4648 decode.

Cosmetic fixes (tests already pass; fixtures were stale)

3. stats.md — pre-2.1 stats shape (RSC6a/returns-paginated-stats-0)
Replaced the deleted all nested-counter blocks with the TS12r entries Dict<String, Int> map and added assertions on entries so TS12r is actually exercised (it previously had no unit coverage).

4. types/options_types.md — pre-REC1 hostnames (TO/endpoint-affects-host-0)
Updated the "Expected Rest Host" table to the REC1a/REC1b4 forms: main.realtime.ably.net, test.realtime.ably.net, custom-env.realtime.ably.net.

Files changed

• uts/rest/unit/presence/rest_presence.md
• uts/rest/unit/encoding/message_encoding.md
• uts/rest/unit/stats.md
• uts/rest/unit/types/options_types.md

No specifications/ change required.

Merge order

PR 4 of 7. Merge after the publish-body PR (shares encoding/message_encoding.md) and coordinate with the presence PR (shares presence/rest_presence.md).

Closes #528

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