dedup: fix stale valueType in boundedSeriesIterator.Next()

[anusha19murthy]

What

boundedSeriesIterator.Next() returns a stale valueType when it seeks past samples before mint.

When the first sample falls before mint, the iterator seeks forward correctly but still returns the valueType from the original sample (before the seek) instead of the one it landed on. If a series changes from float to histogram right at the query boundary, we end up returning ValFloat when the actual sample is a histogram.

The fix

I capture the return value of Seek() back into valueType so the caller gets the correct type:

// before
if it.Seek(it.mint) == chunkenc.ValNone {

// after
valueType = it.Seek(it.mint)
if valueType == chunkenc.ValNone {

This was introduced in 789046ad1 when the return type changed from bool to chunkenc.ValueType — the old bool version couldn't have this mismatch.

Tests

Added TestBoundedSeriesIteratorNextValueType with a mixedTypeIterator helper that supports interleaved float/histogram samples. Covers:

• float before mint, histogram at mint (the actual bug)
• float before mint, float at mint
• sample already within bounds
• all samples before mint
• sample beyond maxt

[GiedriusS]

Why not add mixed type support to type sample?

[anusha19murthy]

I've added the required changes

[GiedriusS]

Linter errors seem related

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.54%. Comparing base (050d688) to head (871c6a7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9028      +/-   ##
==========================================
- Coverage   64.55%   64.54%   -0.02%     
==========================================
  Files         289      289              
  Lines       37412    37413       +1     
==========================================
- Hits        24153    24149       -4     
- Misses      11158    11164       +6     
+ Partials     2101     2100       -1     

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