fix(frontend): apply validated max-nodes default to v1 flame graph output

[1linkovdim]

Problem

On the v1 read path, SelectMergeStacktraces and SelectMergeSpanProfile validate max-nodes (applying the per-tenant max_flamegraph_nodes_default and clamping to max_flamegraph_nodes_max) and use the validated value for the per-query fan-out — but then build the final merged flame graph / tree with the raw request value c.Msg.GetMaxNodes().

When a client omits maxNodes (e.g. Grafana Explore Profiles, which only sends the parameter when it's set), the raw value is 0. Tree.minValue treats maxNodes < 1 as "no limit", so the final flame graph is emitted untruncated:

• the configured default (max_flamegraph_nodes_default) is ignored, and
• the max ceiling is bypassed — the merged tree can exceed max_flamegraph_nodes_max, because that limit lives inside ValidateMaxNodes, which the final assembly skips.

User-visible effect

An omitted maxNodes renders a far larger tree than an explicit one. So on a broad/deep query (worst on transformed-profiles, where the injected service-name root makes a very wide tree), the top-table's other bucket grows when you raise maxNodes toward the configured default — the opposite of what you'd expect — because the "default" view is actually unbounded.

Repro on a fleet query ({"nf.app"=~".*dgw.*"}, transformed-profiles): omit → 234k-node tree / 6.4M other; explicit 16384 → 26k nodes / 8.7M other; explicit 65536 → 7.67M; 131073 → correctly rejected. Total samples conserved throughout — it's purely truncation of the final assembly.

Fix

• selectMergeStacktracesTree now returns the validated maxNodes; SelectMergeStacktraces uses it for both NewFlameGraph and Tree.Bytes.
• SelectMergeSpanProfile uses its in-scope validated maxNodes for the final assembly.
• Diff was already correct (it passes the validated maxNodes to NewFlamegraphDiff); its call sites were updated for the new helper signature.

The v2 read path already threads the validated value through TreeQuery, so this aligns v1 with v2.

Tests

Adds Test_Frontend_SelectMergeStacktraces_MaxNodesDefault:

• omitted maxNodes truncates to the configured default (produces an other node, bounded node count, not the full tree);
• an explicit value above the max is rejected.

Verified the test fails without the fix and passes with it. go test ./pkg/frontend/... ./pkg/model/ all green; gofmt clean.

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