fix(common): 🛡️CWE-681 & CWE-197 eliminate tainted numeric casts in size parsing by using BigDecimal with range guards (#5521)

# Description of Changes

This pull request refactors and improves the logic for converting
human-readable size strings (like "10MB", "2.5GB") to bytes in the
`GeneralUtils` utility class. The main enhancement is switching from
imprecise floating-point arithmetic to `BigDecimal` for more accurate
and robust conversions, and centralizing the conversion logic to reduce
code duplication and improve maintainability.

**Improvements to size conversion logic:**

* Replaced all floating-point arithmetic in `convertSizeToBytes` with
`BigDecimal` operations to ensure precision and to handle large values
more safely.
* Introduced a new private method `toBytes(BigDecimal value, int
powerOf1024)` to centralize and standardize the conversion from size
units to bytes, including error handling for negative and excessively
large values.
* Added constants `KIB` and `LONG_MAX_DECIMAL` for improved readability
and maintainability of size calculations.
* Added a helper method `parseSizeValue(String value)` to consistently
parse size values as `BigDecimal`.
* Updated imports to include `BigDecimal` and `RoundingMode` for the new
conversion logic.

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

app/common/src/test/java/stirling/software/common/util/GeneralUtilsAdditionalTest.java

@@ -16,6 +16,14 @@ class GeneralUtilsAdditionalTest {
         assertNull(GeneralUtils.convertSizeToBytes(null));
     }
 
+    @Test
+    void testConvertSizeToBytesEdgeCases() {
+        assertNull(GeneralUtils.convertSizeToBytes("-10MB"));
+        assertNull(GeneralUtils.convertSizeToBytes("10000000TB")); // overflow beyond long
+        assertEquals(1099511627776L, GeneralUtils.convertSizeToBytes("1TB"));
+        assertEquals(2684354560L, GeneralUtils.convertSizeToBytes("2.5GB"));
+    }
+
     @Test
     void testFormatBytes() {
         assertEquals("512 B", GeneralUtils.formatBytes(512));