Handle setting RichMultiString keys to null

[myieye]

Resolves #1783

Thoughts and questions:

• It's not clear to me if we have "replace with empty string" ops in prod. Currently for both multi-strings and rich-multi-strings, empty values' keys always just get removed on deserialization.
• It's sort of breaking the .NET contract to remove keys on a set, but it keeps our model/behaviour more consistent. Converting nulls to empty rich-strings would be comparably weird.
• Add operations have the same problem, but...there are probably no changes out there that would trip over that. Should we extend those methods to cover the null-case too anyway?

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📝 Walkthrough

Walkthrough

The changes introduce explicit handling for null values in the RichMultiString dictionary setter, treating them as removal requests rather than causing exceptions. Additionally, a new unit test verifies that applying a JSON patch with an empty string properly empties the RichMultiString instance.

Changes

Files/Paths	Change Summary
backend/FwLite/MiniLcm/Models/RichMultiString.cs	Modified the IDictionary.this[object key] setter to treat null as a remove operation.
backend/FwLite/MiniLcm.Tests/RichMultiStringTests.cs	Added a unit test to verify JSON patch replaces with empty string result in an empty object.

Assessment against linked issues

Objective (Issue #)	Addressed	Explanation
Ensure that a JSON patch operation with an empty string is interpreted as a remove (i.e., does not throw, but removes the entry) (#1783)	✅
Prevent exceptions when deserializing or patching with an empty string/null in RichMultiString (#1783)	✅

Assessment against linked issues: Out-of-scope changes

No out-of-scope changes found.

Possibly related PRs

• change RichMultiString.cs to store RichStrings instead of just strings #1692: Modifies RichMultiString and its tests to handle RichString values and updates JSON patch handling, directly related to the changes in this PR.

Poem

A patch for strings both rich and bright,
Now empty means “remove”—that’s right!
No more exceptions in the night,
Our tests confirm it works just right.
🐇✨
Code hops ahead, with bugs in flight!

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[argos-ci]

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Build	Status	Details	Updated (UTC)
default (Inspect)	✅ No changes detected	-	Jul 1, 2025, 6:50 AM

[github-actions]

UI unit Tests

 1 files  ± 0  31 suites  +5   9s ⏱️ -1s
36 tests +10  36 ✅ +10  0 💤 ±0  0 ❌ ±0 
64 runs  +12  64 ✅ +12  0 💤 ±0  0 ❌ ±0 

Results for commit b20f97b. ± Comparison against base commit 492fe8b.

♻️ This comment has been updated with latest results.

[hahn-kev]

nice catch. I agree it is weird, but I'm ok with it here. I'd be fine with doing the same on Add, but also fine to just ignore it for now. I did introduce a method on WSId which probably should have been there way sooner.

[myieye]

Test failures are already in develop