chore(tailer): fix a flaky tests in tail/querier_test.go

[ashwanthgoli]

What this PR does / why we need it:

Mock TailDisconnectedIngesters calls to return empty clients map. This func gets called periodically from the tailer's loop, not implementing it could result in a panic

TestQuerier_concurrentTailLimits/ring_containing_one_active_ingester_and_0_active_tailers (0.00s)
panic:
        assert: mock: I don't know what to return because the method call was unexpected.
                Either do Mock.On("TailDisconnectedIngesters").Return(...) first, or remove the TailDisconnectedIngesters() call.
                This method was unexpected:
                        TailDisconnectedIngesters(*context.valueCtx,*logproto.TailRequest,[]string)
                        0: &context.valueCtx{Context:context.backgroundCtx{emptyCtx:context.emptyCtx{}}, key:0, val:"test"}
                        1: &logproto.TailRequest{Query: "{type=\"test\"}",
        DelayFor: 0x0,
        Limit: 0xa,
        Start: time.Date(2025, time.June, 30, 13, 10, 45, 569837000, time.Local),
        Plan: &plan.QueryPlan{AST:(*syntax.MatchersExpr)(0x14000e00c48)},
        }
                        2: []string{"ingester-1"}
                at: [/Users/grafana/Workspace/loki/pkg/querier/tail/tail_mock_test.go:29 /Users/grafana/Workspace/loki/pkg/querier/tail/querier.go:134 /Users/grafana/Workspace/loki/pkg/querier/tail/tail.go:189 /Users/grafana/Workspace/loki/pkg/querier/tail/tail.go:94 /opt/homebrew/Cellar/go/1.24.1/libexec/src/runtime/asm_arm64.s:1223]

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
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