Implement AccessoryDecimalIntegerStringType::tryRemove() and preserve accessories when re-unioning '0'

[phpstan-bot]

Summary

Removing the constant string '0' from a decimal-int-string did not change the
type — PHPStan still considered the result possibly falsy, even though "0" is the
only falsy decimal integer string. After if ($s !== '0') (or any truthy
narrowing), $s should be decimal-int-string&non-falsy-string. This PR makes that
narrowing work and also fixes the inverse simplification so unioning '0' back in
collapses to the original type.

Changes

• src/Type/Accessory/AccessoryDecimalIntegerStringType.php: dropped
  NonRemoveableTypeTrait and implemented tryRemove(). For a non-inverse
  decimal-int-string, removing the constant '0' returns
  string & decimal-int-string & non-falsy-string. The check uses
  Type::getConstantStrings() rather than instanceof ConstantStringType.
• src/Type/TypeCombinator.php: replaced the describe()-based '0' | …
  union-simplification branches with a new downgradeNonFalsyStringToNonEmpty()
  helper that preserves all accessory refinements (numeric-string,
  decimal-int-string, lowercase-string, …) while downgrading the non-falsy flag to
  non-empty, only adding AccessoryNonEmptyStringType when no remaining accessory
  already implies non-emptiness. Bumped the corresponding instanceof IntersectionType
  baseline count.
• tests/PHPStan/Analyser/nsrt/decimal-int-string.php: added regression assertions.

Root cause

Accessory string types encode refinements that interact with falsy-ness.
AccessoryNonEmptyStringType and AccessoryNumericStringType already convert to a
non-falsy variant when '0' is removed (their tryRemove()), but
AccessoryDecimalIntegerStringType used NonRemoveableTypeTrait and so silently
ignored the removal — the same family bug, one sibling left unfixed.

The mirror operation (union) had a parallel gap: compareTypesInUnion() only
recognized the exact non-falsy-string description when folding '0' back in, so
any non-falsy string that also carried another accessory
(numeric-string&non-falsy-string, the decimal-int variant, lowercase, …) was left
as an un-simplified '0' | (…&non-falsy-string) union. Matching on describe() is
exactly the fragile pattern called out in CLAUDE.md; the new helper works
structurally on the intersection members instead.

Test

tests/PHPStan/Analyser/nsrt/decimal-int-string.php now asserts:

• decimal-int-string narrows to decimal-int-string&non-falsy-string after
  !== '0', != '0', and truthy checks; the negative branch is '0'.
• non-decimal-int-string is unchanged by removing '0' (it never contains '0').
• '0' | … round-trips: decimal-int-string, numeric-string and
  non-empty-string each collapse back to themselves after $x !== '0' ? $x : '0'.

All three round-trip and removal assertions fail on 2.2.x without the source
changes and pass with them. Full make tests and make phpstan are green.

Fixes phpstan/phpstan#14785

[staabm]

@VincentLanglet please have a look when time allows :)

[staabm]

CI errors are pre-existing