Add exact Redis Stream trimming support

[derekste]

Summary

• add an RA_ArgsAdd::approximateTrim option for Redis Stream writes
• preserve the existing approximate MAXLEN ~ behavior by default
• allow callers with a hard memory or entry-count contract to request exact
  MAXLEN = trimming
• cover exact retention in the Redis-backed test suite and document the new
  argument

Motivation

Image streams can contain multi-megabyte entries, so Redis's approximate trim
may materially exceed a configured memory budget. The AreaDetector Redis data
plane uses this option to retain exactly 16 recent frames while keeping the
existing behavior unchanged for all current callers.

Verification

• the new test writes ten values with a retention of three and verifies the
  exact final entries
• ADRedis builds against this branch in the full ADAravis IOC image
• the deployed GE1350 stream remained exactly 16 entries during a 600-frame
  real-camera comparison and concurrent PVA/video fanout

[Copilot]

Pull request overview

This PR adds a caller-controlled switch to choose between Redis Stream approximate trimming (MAXLEN ~) and exact trimming (MAXLEN =) when writing stream entries via RedisAdapter, while keeping approximate trimming as the default for backward compatibility and throughput.

Changes:

• Extend RA_ArgsAdd with approximateTrim (default true) to allow opting into exact MAXLEN = trimming.
• Thread the new option through all stream-write paths (addSingleValue, addSingleDouble, addSingleList).
• Add a Redis-backed test validating exact retention behavior and document the new argument in the API docs.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test.cpp	Adds ExactTrim test validating strict retention when approximateTrim=false.
RedisAdapterTempl.hpp	Passes approximateTrim through templated stream write helpers to xaddTrim.
RedisAdapter.hpp	Adds RA_ArgsAdd::approximateTrim and wires it through addSingleList overloads + helper declaration.
RedisAdapter.cpp	Wires approximateTrim through the addSingleDouble stream write path.
mock/MockRedisAdapter.hpp	Keeps mock RA_ArgsAdd in sync with the real adapter’s argument struct.
docs/api.md	Documents the new RA_ArgsAdd.approximateTrim behavior and default.

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