8.8 (unstable) validation

[mgravell]

Back-compat validation for server 8.8; no functional changes - CI only; compensates for:

• changes to some search error messages (no such index becomes index not found, from here)
• changes to info search output (looks like a lot of internal detail sections removed)

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Note

Low Risk
Low risk: changes are limited to CI/docker test configuration and test assertions to accommodate Redis 8.8 output/message differences.

Overview
Adds Redis 8.8 to the GitHub Actions integration test matrix and updates the docker test setup to use the redislabs/client-libs-test:unstable-23321515778-debian image (with a new tests/dockers/.env.v8.8).

Adjusts a few tests to be version-aware for Redis 8.8, including new EndpointsFixture.IsAtLeast(...) helper and updated assertions for INFO search output size and FT.DROPINDEX error message text.

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[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

There are 2 total unresolved issues (including 1 from previous review).

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