feat(spanner): accept T: Into<Value> in parameter and mutation bindings

[olavloite]

Changes StatementBuilder::add_param, StatementBuilder::add_typed_param, and ValueBinder::to to accept T: Into<Value> instead of &T: ToValue.

This allows callers to pass owned values directly (like String, Value, or Option<T>) zero-copy without unnecessary cloning, while maintaining complete backward compatibility for references (&str, &i64, &Option<T>).

Key Changes

• Direct From<T> for Value Implementations: Added zero-copy From conversions for owned types (String, i64, i32, bool, f64, f32, Decimal, Date, OffsetDateTime, SystemTime, wkt::Timestamp, Vec<u8>, Option<T>, Vec<T>, ProtoValue).
• Decoupled ToValue: All ToValue trait implementations now delegate directly to From / .into(), decoupling conversion logic from ToValue and paving the way for eventual deprecation of the ToValue trait.
• Unsized Slice Support ([u8] and str): Implemented ToValue for [u8] and str (and &[u8], &str), enabling zero-copy slice conversions and generic trait bounds T: ToValue + ?Sized. This supersedes both PR perf(spanner): add ToValue for &[u8] #6070 and PR feat(spanner): implement ToValue for str #6087.
• Untyped NULL Support: Added Value::null(), From<()> and ToValue for () to easily support untyped NULL parameters (Value::null(), None::<()>, None::<Value>).
• Cleaned up Examples & Tests: Updated example scripts and tests to pass owned values directly, eliminating needless reference borrows (clippy::needless_borrows_for_generic_args).

BREAKING CHANGE

BREAKING CHANGE: This is a minor breaking change only for code that passes inline &None with a method-level turbofish:

// Old (fails to compile):
builder.set("Col").to::<Option<bool>>(&None);

// Works both before and after this change:
builder.set("Col").to(&None::<bool>);

// New (remove the &):
builder.set("Col").to::<Option<bool>>(None);
// Or:
builder.set("Col").to(None::<bool>);
builder.set("Col").to(Value::null());

Code passing existing Option variables (like .to(&my_var)) is unaffected and continues to compile unchanged.

---

This pull request replaces the following pull requests:

• perf(spanner): add ToValue for &[u8] #6070
• perf(spanner): add by-value parameter and mutation binding methods #6071
• feat(spanner): implement ToValue for str #6087

[gemini-code-assist]

Code Review

This pull request refactors the Spanner client's parameter and mutation value binding APIs (ValueBinder::to, StatementBuilder::add_param, and StatementBuilder::add_typed_param) to accept values implementing Into<Value> by value, rather than requiring references to types implementing ToValue. This change significantly simplifies client code by eliminating the need for explicit borrows (e.g., .to(&1) becomes .to(1)). Corresponding updates have been made across examples, integration tests, and unit tests, and new tests have been added to verify various value conversions, including owned, borrowed, optional, and null types. No review comments were provided, so there is no additional feedback to address.

[codecov]

Codecov Report

❌ Patch coverage is 98.99749% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.58%. Comparing base (b3a930b) to head (b205c13).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spanner/src/to_value.rs	98.39%	3 Missing ⚠️
src/spanner/src/mutation.rs	98.83%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #6184    +/-   ##
========================================
  Coverage   96.57%   96.58%            
========================================
  Files         264      264            
  Lines       66577    66894   +317     
========================================
+ Hits        64297    64609   +312     
- Misses       2280     2285     +5     

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