Follow-up fixes for the combined Matrix crypto salvage (#71073,
#71543, #71547):
1. Construct _pickle_key from client.device_id (resolved from whoami)
instead of self._device_id (the configured value). Without this, when
#71543 makes the token's real device win over a stale
MATRIX_DEVICE_ID, the pickle key is built from the stale value and
the Olm account is stored under a key that can never be looked up
again — perpetuating the same decryption failure the PRs aim to fix.
2. Skip _migrate_legacy_crypto_pickle when the store was just deleted
by _reset_crypto_store_if_device_changed — there is no account to
migrate. Also check the migration return value and log a warning on
failure instead of proceeding to olm.load() which fails with a
cryptic BAD_ACCOUNT_KEY.
3. Add a local dict cache (_enc_info_cache) to _CryptoStateStore so the
homeserver fallback in get_encryption_info() does not make a network
round-trip on every is_encrypted() call. MemoryStateStore does not
implement set_encryption_info, so the existing cache-back is a no-op.
4. Log homeserver encryption-info query failures at DEBUG level instead
of silently returning None (which would cause OlmMachine to treat an
encrypted room as unencrypted).
5. Update _CryptoStateStore docstring to mention the homeserver fallback.
648c01c693