fix(pipe): avoid nil cache panic with disabled static-TTL multi-cache

[kurochan]

Summary

Fix a nil pointer panic when DoMultiCache is used with multiple
ToStaticTTL() commands while client-side caching is disabled.

Root cause

With DisableCache: true, the pipe does not initialize p.cache.

The DoMultiCache fallback converted Cacheable commands to Completed
commands but preserved the internal static-TTL flag. In the background read
loop, the second and subsequent replies then entered the static-TTL path and
called p.cache.Update or p.cache.Cancel, causing a nil pointer panic.

Changes

• Clear static-TTL metadata before forwarding disabled-cache requests to
  DoMulti.
• Guard the background static-TTL path when no cache store exists.
• Add regression tests for successful replies and Redis errors with multiple
  static-TTL commands and a cancelable context.

Compatibility

• DisableCache remains equivalent to Do/DoMulti.
• Client-side caching behavior is unchanged when enabled.
• Redis errors are returned to callers unchanged.

---

Note

Low Risk
Narrow fix for DisableCache + static-TTL edge case; enabled client-side caching paths are unchanged aside from the same clear-on-fallback in DoCache.

Overview
Fixes a nil pointer panic when client-side caching is off (DisableCache: true) but callers still use DoCache / DoMultiCache with ToStaticTTL() commands.

With no cache store, those APIs already fall back to plain Do / DoMulti, but the static-TTL flag stayed on the wire commands. The background read loop then treated later replies as cacheable and called p.cache.Update / Cancel on a nil store—especially on the second+ command in a DoMultiCache batch.

Change: strip static-TTL metadata via cmds.ClearStaticTTL before forwarding in both disabled-cache paths so behavior matches untagged Do/DoMulti. Regression tests cover multi-command success, mixed Redis errors, and single DoCache.

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[rueian]

Do we need to ClearStaticTTL at DoCache as well?