Fix serialization round trip for QuantizationType::SQ without standard deviation

[Alex Razumov (arrayka)]

Why

QuantizationType::SQ with no explicit standard deviation did not round-trip through serialization:

QuantizationType::SQ {
    nbits: <N>,
    standard_deviation: None,
}

It serialized as SQ_N_None, which the deserializer rejected.

What

Serialize SQ configs without an explicit standard deviation as SQ_N and cover the case in round-trip tests.

[Copilot]

Copilot review overview

🟢 Approval recommended

The serialization format now matches the existing parser and is adequately tested.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes SQ_N serialization so scalar quantization configurations without an explicit standard deviation round-trip correctly.

Changes:

• Serializes SQ without standard deviation as SQ_N.
• Adds formatting and serialization round-trip coverage.

File	Description
diskann-disk/​src/​build/​configuration/​quantization_types.rs	Aligns SQ formatting with parsing and updates tests.

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[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.86%. Comparing base (31589c3) to head (90ece37).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1461      +/-   ##
==========================================
- Coverage   91.90%   90.86%   -1.04%     
==========================================
  Files         582      582              
  Lines      114947   114950       +3     
==========================================
- Hits       105640   104452    -1188     
- Misses       9307    10498    +1191     

Flag	Coverage Δ
miri	90.86% <100.00%> (-1.04%)	⬇️
unittests	90.63% <100.00%> (-1.22%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...disk/src/build/configuration/quantization_types.rs	98.14% <100.00%> (+0.03%)	⬆️

... and 47 files with indirect coverage changes

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