fix: add fetch_secrets support for ai-rate-limiting, limit-conn, and limit-req plugins

[shreemaan-abhishek]

Description

While testing distributed rate limiting with secret references, I noticed that ai-rate-limiting, limit-conn, and limit-req were passing literal strings such as "$ENV://REDIS_HOST" to Redis instead of resolving them, even though limit-count already supported this. The root cause is that those three plugins never invoked apisix.secret.fetch_secrets on their config before using it. This change adds the missing fetch_secrets calls in the relevant access/log/instance-status entry points so that $ENV:// and $secret:// references in fields like redis_host are resolved consistently across all rate-limiting plugins.

Which issue(s) this PR fixes:

Fixes #

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[nic-6443]

@shreemaan-abhishek We have a new plan for the secret feature #13312