fix(query-engine): make heap top-k keys collision-safe

[milindsrivastava1997]

Parent

#678

What to build

Replace or harden delimiter-based label-key encoding used by CountMinSketchWithHeap. Semicolon-joined keys cannot safely represent label values containing semicolons and can decode to the wrong label vector.

Preserve compatibility with existing persisted data if required.

Acceptance criteria

• Distinct label vectors cannot encode to the same heap key.
• All supported label values round-trip losslessly.
• Existing persisted keys have a documented migration or compatibility strategy.
• Tests cover semicolons, empty values, and multiple labels.

Blocked by

None - can start immediately.

Concrete example

Current encoding:

let encoded = labels.join(";");
let decoded = encoded.split(';').collect::<Vec<_>>();

These distinct label vectors collide:

["a;b", "c"] -> "a;b;c"
["a", "b;c"] -> "a;b;c"

A value such as path="a;b" also decodes as two labels instead of one.

The replacement should use a structured or length-prefixed representation, for example:

[3]["a;b"][1]["c"]