[mrattle]

Summary

Single-server set_multi (the PipelinedSetter → write_multi_storage_req fast path) built keys differently from every other operation:

• It dropped the base64 flag returned by KeyRegularizer.encode, so keys containing whitespace or non-ASCII characters were stored under their base64 text. get and get_multi never find them there.
• It applied the namespace and truncation to the encoded key (Fix set multi namespace on single server #47) instead of the raw key.
• It skipped to_s, so Symbol and Integer keys raised NoMethodError.

Multi-server rings were unaffected because they fall back to quiet { set }.

This moves key validation into PipelinedSetter, using the client's KeyManager in the same way as PipelinedGetter and PipelinedDeleter. write_multi_storage_req now only encodes the key and sends b. The per-server KeyManager added to Protocol::Base for this path had no other users and is removed.

Intended behaviour:

• set_multi writes to the same keys that set, get, and get_multi use, for any ring size, with or without a namespace. Items already written under base64 text are never read and expire normally.
• set_multi accepts the same non-String keys as set.
• Blank and nil keys raise ArgumentError before any I/O, as with set. Previously nil raised NoMethodError, and a blank key failed inside the request, which closed the connection.

Not in scope: keys that need base64 are still limited by KeyManager truncating at 250 raw characters, while memcached caps the encoded key at 250 characters. That will be addressed separately.

Testing

• New tests in test_namespace_and_key.rb call set_multi, then get and get_multi, on 1- and 2-server rings, with and without a namespace, for plain, space, tab, and non-ASCII keys. They also cover Symbol and Integer keys on both ring sizes. The three single-server cases fail on main.
• These replace the two set_multi namespace tests from Fix set multi namespace on single server #47; the one named "multiple servers" ran on a single server without a namespace. The single-server concurrency test uses a namespace again, and its stale bug note is removed.
• Full suite passes locally on Ruby 4.0.7 / memcached 1.6.45, excluding test_rack_session.rb, which does not load on Ruby 4.0 with or without this change (rack 2.2 requires the removed cgi/cookie).