Chg #960: Pass non-null and non-expression values only to dbTypecast()

[KalimeroMK]

Fix #960

Changes

• Adds the Yiisoft\Db\dbTypecast() function (proposed in the issue) that returns null values as is, and delegates everything else to ColumnInterface::dbTypecast().

• Internal call sites (AbstractDMLQueryBuilder, AbstractColumnDefinitionBuilder, DateTimeValueBuilder) now use the function.

• Column implementations are intentionally left untouched. Two findings from the driver test matrices:

  • Expression is Stringable, so removing the ExpressionInterface arms silently stringifies expressions instead of passing them through (breaks DDL builders in db-pgsql/db-mssql that pass Expression default values directly).
  • Some driver implementations process expressions: Yiisoft\Db\Mssql\Column\BinaryColumn::dbTypecast() unwraps Param values into CONVERT(VARBINARY(MAX), ...) expressions, so a wrapper must not filter ExpressionInterface either.

  Net effect: only null is safe to pre-filter. Simplifying the implementations needs a coordinated change across the driver packages and can be done as a follow-up for 3.0.

Benchmark

2M calls per round, median of 7 rounds, PHP 8.5 with OPcache + JIT (tracing), workload: 80% scalars, 10% null, 10% expressions, IntegerColumn + StringColumn:

approach	time per round
current master (direct call)	~34 ms
this PR (dbTypecast() function)	~42 ms
check inlined at the call site	~34.5 ms

The function adds ~4 ns per call on trivial casts. In the real DML path, buildValue() dominates the per-value cost, so the end-to-end impact is within noise; the same applies to AR inserts/updates where network I/O prevails. The numbers are here so the trade-off is explicit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.62%. Comparing base (f0b0d2b) to head (fb89316).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1197   +/-   ##
=========================================
  Coverage     98.62%   98.62%           
  Complexity     1645     1645           
=========================================
  Files           120      121    +1     
  Lines          4288     4290    +2     
=========================================
+ Hits           4229     4231    +2     
  Misses           59       59           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.