feat: add hash field TTL commands (HPERSIST, HTTL, HPTTL, HSETEX)

[adamrothman]

I'm working on a project that makes use of TTLs for hash values. I'd like to be able to write tests for this functionality, but miniredis currently lacks support for most of the necessary commands. So I put Claude to work!

This PR doesn't include all of the TTL-related commands, but it's a start. If there's an appetite for the rest, I can see about implementing those too.

Summary

Adds wire commands and direct API methods for hash field TTL operations, building on the existing HEXPIRE implementation:

• HPERSIST — remove expiration from hash fields
• HTTL — get remaining TTL of hash fields in seconds
• HPTTL — get remaining TTL of hash fields in milliseconds
• HSETEX — set hash fields with optional expiration (supports EX, PX, EXAT, PXAT, KEEPTTL, FNX, FXX)
• Direct API methods — HExpire, HPersist, HTTL, HSetEX on both *Miniredis and *RedisDB
• Extracted shared parseFieldsArgs helper for the FIELDS numfields field... argument block

Test plan

• Unit tests for each new wire command (HPERSIST, HTTL, HPTTL, HSETEX)
• Unit tests for direct API methods (TestDirectHashFieldTTL)
• Integration tests for all new commands
• Regression tests — all existing hash tests continue to pass
• Full test suite passes (go test ./... -count=1)
• go vet ./... clean

[alicebob]

Thanks for the PR! It looks good, but I'll have to go through it in detail, which will take a bit.