Increase test coverage for core vector classes to 100%

[Copilot]

Test coverage for core vector classes (Vector, HalfVector, Bit, SparseVector) was at 66-82%, with critical database conversion methods and edge cases untested.

Changes

Database Adapter Coverage

• Added tests for _to_db(), _to_db_binary(), _from_db(), _from_db_binary() methods
• Covers None handling, dimension validation, type conversion paths

Serialization Coverage

• Added to_text(), from_text(), to_binary(), from_binary() tests for Bit class
• Added roundtrip serialization tests for all classes

Edge Cases

• Empty vectors, single elements, negative values
• Type inequality comparisons (e.g., Vector([1,2,3]) != [1,2,3])
• Sparse arrays with invalid dimensions
• Float precision handling

Error Conditions

• Dimension mismatch in _to_db(dim=N)
• Invalid type errors in Bit _to_db() methods
• Invalid binary format errors

Coverage Impact

Class	Before	After
Vector	66%	100%
HalfVector	66%	100%
Bit	72%	100%
SparseVector	82%	99%

Test count: 58 → 135 (+77 tests)

Note: SparseVector's missing 1% is scipy version compatibility code at line 90 (fallback for scipy <1.13).

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Correggi i tests in questo repo</issue_title>
<issue_description>I test in questo repo non hanno una coverage molto alta, puoi aumentarla o comunque aggiungere dettagli a quelli gia' presenti. </issue_description>

<agent_instructions>just do it</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Correggi i tests in questo repo #3

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.