Extends the MSET pre-sizing (phpredis#2861) to the shared multi-argument builders
so a high-arity or large-payload call doesn't regrow the smart_string a
4KB page at a time through its append loop. A small shared helper,
redis_zval_frame_est, returns an upper-bound RESP frame size per argument
(framing overhead plus the payload length when it's already a string;
non-strings get a fixed allowance). Each builder sums it over its args
in a cheap read-only pre-pass and reserves once.

Covers redis_key_varval_cmd (SADD/SREM/RPUSH/LPUSH/ZREM/...),
gen_key_arr_cmd (their array forms), gen_vararg_cmd, redis_mget_cmd, and
redis_hmset_cmd. The estimate only reads types and lengths, so it cannot
change the emitted bytes; under-estimating just falls back to growth.

gen_varkey_cmd (DEL/SUNION/SINTER/...) is left for a follow-up: its
single-array/timeout branching makes a clean reservation point less
obvious.

Verified on PHP 8.4: 1000-member SADD/RPUSH/sAddArray, 50KB-blob RPUSH,
800-key MGET, 400-field HMSET incl binary fields, prefix and PHP
serializer combinations; no leaks under report_memleaks; 25 project
suite methods pass.