Fix flow-state merge freeze on repeated PROGRESS snapshots

[rowanadair-r7]

Problem

mergeRecords() in services/launcher/flows.go only replaces the accumulated
QueryStats when the incoming stats record represents a genuine transition
from running to a terminal state:

if len(stats_context.QueryStats) > 0 {
    if len(collection_context.QueryStats) == 0 ||
        is_running(collection_context) && !is_running(stats_context) {
        collection_context.QueryStats = stats_context.QueryStats
    }
}

Two consecutive still-running (PROGRESS) snapshots never update it. If a
flow's query never reaches a terminal state (e.g. it's stuck on a slow
upload), the collection context is permanently pinned at the very first
progress snapshot it ever received - even though the client keeps sending
fresher progress for as long as the query runs. Since UpdateFlowStats
derives TotalUploadedBytes, TotalCollectedRows, TotalLogs, State, etc.
purely from QueryStats, the whole flow view gets stuck showing stale/empty
progress and never reflects what the client has actually done.

Fix

Each FlowStats message the client sends already contains the complete,
cumulative status of every query in the flow so far (built by iterating the
full responders slice - see responder.FlowContext.getStats() upstream),
never a partial delta. So it's always safe to adopt the latest snapshot
wholesale as long as the accumulator hasn't reached a terminal state yet:

if len(stats_context.QueryStats) > 0 {
    if len(collection_context.QueryStats) == 0 ||
        is_running(collection_context) {
        collection_context.QueryStats = stats_context.QueryStats
    }
}

This still freezes correctly once the accumulator reaches a terminal state
(so a stray/out-of-order message arriving after completion doesn't reopen
it), it just stops discarding fresher in-progress snapshots.

Testing

Added TestFlowStatsProgressNotFrozen in ingestion/ingestor_test.go,
which sends two consecutive PROGRESS snapshots via
Ingestor.HandleFlowStats with increasing row/byte counts and asserts
GetFlowDetails reflects the second, fresher snapshot rather than staying
pinned at the first. Verified it fails against the old logic and passes with
the fix. Also ran the full existing suite (go test -p 1 ./...) - no
regressions; TestLauncher and TestListDirectory are unaffected since
they only exercise a single running->terminal transition, where old and new
logic agree.

[CLAassistant]

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