fix(sdk): improve force flush logic

[grvmishra788]

Description

Fixes #4631

Before: SynchronousMultiSpanProcessor.force_flush and SynchronousMultiLogRecordProcessor.force_flush used if not sp.force_flush(...): return False, which stopped iterating through processors as soon as any one returned a falsy value. This meant that a processor returning None would cause all subsequent processors to be silently skipped. The concurrent variants had the same truthiness check on results, incorrectly treating None as failure.

After: All multi-processor force_flush implementations now:

• Continue calling every registered processor regardless of individual return values (timeout-based early exit is preserved).
• Use is False instead of not result to distinguish between an explicit False (timeout exceeded) and None (not implemented / nothing to flush).
• SpanProcessor.force_flush returns True by default instead of implicitly returning None.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Unit tests (SDK)
• Typecheck
• Lint
• Pre-commit
• Spellcheck

Does This PR Require a Contrib Repo Change?

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added

[grvmishra788]

Hi @alexmojaki! Could you please review this one? TIA.