fix(server): preserve existing connections after requirepass changes

[aviralgarg05]

Description

This fixes a Redis Sentinel compatibility issue reported in #3267.

When Kvrocks accepted a connection while requirepass was still empty, it did not immediately mark that connection as authenticated. Instead, it deferred assigning the default namespace/admin context until the connection sent its first command.

That differs from Redis behavior.

In practice, this created a race with older Sentinel versions. Sentinel could establish a connection to the master before auth-pass was configured, but only send its first probe afterward. Redis continues to allow that already-open connection to operate, while Kvrocks responded with NOAUTH. As a result, Sentinel could temporarily treat the master as down and fail to discover replicas until a reset or restart forced a fresh connection cycle.

This change fixes the behavior at the source:

• Passwordless connections are now initialized as authenticated when they are accepted.
• Existing connections opened before requirepass is later set continue to behave like Redis.
• New connections created after requirepass is set still require authentication as expected.

Files changed

• src/server/redis_connection.cc
• tests/gocase/unit/auth/auth_test.go

Why this is safe

The change is limited to the connection initialization path and only affects connections created while the server is running without a password configured.

It does not relax authentication for:

• servers that already start with requirepass set
• connections created after requirepass is enabled

So the fix restores Redis-compatible semantics without changing the normal protected path.

Test coverage

Added a regression test that verifies:

1. a connection opened before CONFIG SET requirepass ... remains usable
2. a new connection opened after requirepass is set receives NOAUTH

Validation

Ran:

• ./x.py build -j 8
• ./x.py format
• ./x.py check format
• go test -timeout=1800s -run 'TestNoAuth|TestAuth' ./unit/auth -binPath=/Users/aviralgarg/Everything/kvrocks/build/kvrocks -cliPath=redis-cli -workspace=/Users/aviralgarg/Everything/kvrocks/tests/gocase/workspace

The auth test suite was run 3 times and passed each time.

I also manually verified the repro condition:

• before the fix, an idle connection created before requirepass was enabled received -NOAUTH
• after the fix, the same connection correctly receives +PONG

Notes

Two repo-wide checks are currently blocked by the local environment rather than this change:

• ./x.py check tidy
  • after wiring run-clang-tidy from local LLVM, it reports existing unrelated tidy failures outside this patch
• ./x.py check golangci-lint
  • local golangci-lint is 2.8.0, while the repo requires 2.9.0+