Fixes set_props to merge instead of overwriting props

[Raahul-Singh]

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Start with a description of this PR. Then edit the list below to the items that make sense for your PR scope, and check off the boxes as you go!

Fixes #3104

Contributor Checklist

• [x] I have broken down my PR scope into the following TODO tasks
  • [x] Fix _set_props inside _make_job_fn to accumulate updates instead of overwriting.
  • [x] Ensure all set_props calls persist across multiple updates.
  • [x] Test multiple set_props calls in a background callback to confirm the fix.
• [ ] I have run the tests locally and they passed. (refer to testing section in contributing)
• [ ] I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• [ ] I have added entry in the CHANGELOG.md
• [ ] If this PR needs a follow-up in dash docs, community thread, I have mentio

ned the relevant URLS as follows - [ ] this GitHub #PR number updates the dash docs - [ ] here is the show and tell thread in Plotly Dash community

This change gets the expected behavior in @faulty13' snippet

https://github.com/user-attachments/assets/1e6d0eb9-6982-4526-85a9-67ba06f4c02c

[BSd3v]

Would be interesting to see if this could take on a behaviour of Patch. Sometimes there may be a reason to append multiple children throughout the span of the entire callback.