redis: a reply nested more than 512 arrays deep fails the command instead of overflowing the stack

[MDA2AV]

A redis server could kill the whole process with one deeply nested reply: the parser's recursion overflowed the reactor thread's stack.

RespProtocol.TryParseAt recursed once per array level, with no bound, on the reactor thread. A reply of one-element arrays nested 100,000 deep is 400 KB on the wire, well inside the 64 MB receive ceiling. It overflowed the stack at about 11,400 levels. A stack overflow cannot be caught in .NET, so the process aborted, taking every reactor with it.

An array nested more than 512 deep now throws a RedisException. That breaks the connection and fails its commands, like any other malformed reply. Real replies nest a few levels, and 512 levels use under 5% of the depth that overflowed. No new state: the depth is a parameter of the recursion.

Tests

New, in RedisReplyTests (no redis needed):

• redis: a reply nested 100,000 arrays deep fails the command instead of overflowing the stack
• control: the same fake server's reply nested 8 arrays deep is parsed

Both PING a FakeServer, a new harness helper that runs a script per accepted loopback connection. Here the script answers with :1 inside nested one-element arrays. The control walks the parsed reply and must report depth 8, so the refusal is about depth and not the fake server.

	main	this PR
reply nested 100,000 deep	process aborts: Stack overflow. (TryParseAt repeated 11,389 times)	RedisException: RESP reply nested more than 512 arrays deep
control: nested 8 deep	depth 8	depth 8

Suites: Unit 63 (1 pending, as on main), Pg 6, Redis 5, with both sidecars running (nothing skipped).

Hot path, barely: one integer compare per array in a reply, and one extra argument.

Seen and left alone, since neither is about depth:

• *<count> allocates new RespValue[count] before any element has arrived, so a 12-byte *100000000\r\n asks for about 3 GB.
• An incomplete array is parsed again from its start on every recv, so a large array that arrives slowly costs quadratic time.

FakeServer.cs is byte-identical to the one in the message-length, SCRAM and open-deadline branches.