[exa-py]: fix Result.__str__ to handle optional text/summary fields

[MiguelAtExa]

Summary

Fixes Result.__str__ to only include text and summary fields when they are not None, matching the behavior of highlights and highlight_scores which already have conditional checks.

Previously, the __str__ method always appended Text: None and Summary: None even when these optional fields weren't requested/returned. This was inconsistent with the type annotations (Optional[str]) and with how exa-js/Vulcan handle these fields (both are optional in the API response schema).

Review & Testing Checklist for Human

• Verify the behavior difference between None (field absent) vs "" (empty string) is acceptable - this PR uses is not None so empty strings will still be printed
• Quick sanity check: run str(Result(url="x", id="y")) and confirm no Text: or Summary: lines appear

Notes

• Verified parity with exa-js SDK where text and summary come from ContentsResultComponent and are conditionally added
• Verified Vulcan types show text: z.string().optional() and summary: z.string().optional()
• Unit tests pass locally

Link to Devin run: https://app.devin.ai/sessions/e66881e28a914d1bae92943ed2f358f2
Requested by: Miguel Brandão (@MiguelAtExa)

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