chore(agent): [Step 1/4] fix PHPStan null-coalesce diagnostics

PLAN_REF: 6e65fe3
PREVIOUS_STEP: 6e65fe3 — initialized release-fix plan

## Completed in This Step
- Removed impossible null-coalesce fallbacks from non-nullable AbstractProvider properties.
- Removed impossible null-coalesce fallback from StatsParser size-unit match offset.
- Verified the CI-equivalent local Composer script passes.

## References Used
- CI run 25469111120 PHPStan output.
- PHPStan nullCoalesce.property and nullCoalesce.offset diagnostics.

## Debts and Warnings
- No behavioral change intended; this is a static-analysis cleanup.
- make precommit skipped because this repository has no Makefile target available.
- desloppify reported 100/100 scores but warned that phpstan tooling was unavailable inside desloppify.

## Progress (ref: 6e65fe3)
- [x] Step 1: Inspect failing code and apply minimal static-analysis fixes. ← this commit
- [x] Step 2: Run analysis and tests.
- [ ] Step 3: Commit the fix.
- [ ] Step 4: Create and push v4.1.1, then monitor CI.

## Next Step
chore(agent): [Step 2/4] create and push v4.1.1

Author: insign

src/Providers/AbstractProvider.php

@@ -120,7 +120,7 @@ protected function validateConfig(array $flags): void
         if (! empty($missing)) {
             throw new InvalidArgumentException(sprintf(
                 'Provider "%s" is missing required configuration: %s',
-                $this->provider ?? 'unknown',
+                $this->provider,
                 implode(', ', $missing)
             ));
         }
@@ -209,7 +209,7 @@ public function hasWarnings(): bool
      */
     public function getRawFlags(): array
     {
-        return $this->flags ?? [];
+        return $this->flags;
     }
 
     /**
@@ -229,7 +229,7 @@ public function extractSecrets(): array
     {
         $secrets = [];
 
-        foreach ($this->flags ?? [] as $key => $value) {
+        foreach ($this->flags as $key => $value) {
             if (! is_string($value) || strlen($value) < 4) {
                 continue;
             }

src/StatsParser.php

@@ -147,7 +147,7 @@ public static function convertSizeToBytes(string $sizeStr): int
         }
 
         $value = (float) $matches[1];
-        $unit = strtoupper(($matches[2] ?? '') === '' ? 'B' : $matches[2]);
+        $unit = strtoupper($matches[2] === '' ? 'B' : $matches[2]);
 
         if (isset($units[$unit])) {
             return (int) ($value * (1024 ** $units[$unit]));