fix(exports): convert bool values for SAV exports

[ukanga]

Changes / Features implemented

• Fixed SAV export generation when a string/text field contains a boolean value.
• Updated encodeifstr() so SAV export values are consistently converted to strings before being passed to savReaderWriter.
• Added regression coverage for boolean values in SAV exports.

Steps taken to verify this change does what is intended

• Confirmed the regression test fails before the fix with:

  text
  AttributeError: 'bool' object has no attribute 'ljust'

• Confirmed the targeted tests pass after the fix:

  python manage.py test \
    onadata.libs.tests.utils.testexporttools.TestExportTools.testencodeifstr \
    onadata.libs.tests.utils.testexportbuilder.TestExportBuilder.testzippedsavexportwithboolstringvalue \
    --settings=onadata.settings.githubactionstest

Side effects of implementing this change

• Boolean values in SAV string fields are now exported as "True" / "False" instead of being passed as raw Python booleans.
• No expected side effects for non-SAV exports.

Before submitting this PR for review, please make sure you have:

• Included tests
• Updated documentation

Closes #3082

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