fix: key builder encoding, reset_settings, and cache_evict guard

[emiliano-go]

Changes

Bug: Key builder ambiguity with special characters in query values

default_key_builder used : as the key segment delimiter and joined query
params with :. Values containing :, =, or & created ambiguous keys
that could not round-trip correctly.

Fix: Changed the query-params segment to use & as a separator and
URL-encode values (%3A for :, %26 for &, %3D for =).

Bug: Missing reset_settings() for testing

get_settings() uses @lru_cache, caching the RedisSettings instance
forever. Tests that mutate environment variables cannot force a fresh reload.

Fix: Added reset_settings() that calls get_settings.cache_clear().
Exported from the public API.

Bug: cache_evict() with no args wipes ALL cache keys

Calling cache_evict() without eviction_group and without key_builder
silently deleted every key under the global prefix.

Fix: Added an early ValueError when both arguments are absent. To
intentionally wipe all keys the caller must provide an explicit
eviction_group.