Fixes set_props to merge instead of overwriting props

[Raahul-Singh]

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

• I have broken down my PR scope into the following TODO tasks
  • Fix _set_props inside _make_job_fn to accumulate updates instead of overwriting.
  • Ensure all set_props calls persist across multiple updates.
  • 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

Screen.Recording.2025-02-21.at.11.36.25.PM.mov

[T4rk1n]

This is only for diskcache, we need the same behavior in regular callbacks, diskcache and celery.

Also need to add tests for the merging.

[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.