Fix Claims.add() default to preserve existing claims instead of replacing them

[Copilot]

Claims.add() was deleting all existing claims with the same property when adding a new claim, contrary to user expectations. The default behavior was REPLACE_ALL, marking existing claims for removal on write.

Changes

• Changed default action_if_exists parameter in Claims.add() from REPLACE_ALL to APPEND_OR_REPLACE
• Added test coverage for the issue scenario and different action_if_exists behaviors

Impact

Before:

item = wbi.item.get('Q105624761')
item.claims.get('P1476')  # Returns 2 existing titles (de, fr)

title_en = MonolingualText(text='English Title', language='en', prop_nr='P1476')
item.claims.add(title_en)
item.write()

# Result: Only English title exists, German and French deleted ❌

After:

item = wbi.item.get('Q105624761')
item.claims.get('P1476')  # Returns 2 existing titles (de, fr)

title_en = MonolingualText(text='English Title', language='en', prop_nr='P1476')
item.claims.add(title_en)
item.write()

# Result: All 3 titles exist (de, fr, en) ✅

Users wanting replace-all behavior can explicitly specify action_if_exists=ActionIfExists.REPLACE_ALL. This aligns Claims.add() with BaseEntity.add_claims(), which already defaulted to APPEND_OR_REPLACE.

Warning

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Adding claim deletes other claims?</issue_title>
<issue_description>I'm trying to add a Claim to a wikidata item, but instead of adding it and keeping all other claims with same property, the tool deletes all other claims and replaces them.

Specifically,
flametti = wbi.item.get(entity_id='Q105624761')
flametti.claims.get(PROPS['title'])
shows the entity has several claims about regarding title, two of them in German, but none in English (yet) so I'd like to add it:
title_en_string = datatypes.MonolingualText(text='Flametti, or The Dandyism of the Poor', language='en', prop_nr=PROPS['title'])
flametti.claims.add(title_en_string)
If I do flametti.claims.get(PROPS['title']) indeed the local object it has four or so titles, now, as it should.
But after
flametti.write(login=login_instance)
the entity ends up having only one title, the one I just added. Is this expected behaviour? What am I doing wrong?</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Adding claim deletes other claims? #922

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