Tighten DecodeDictionary constraint to IDictionary

[pepone]

Fixes #4519.

[Copilot]

Pull request overview

This PR updates the Slice and Ice dictionary decoding APIs to require concrete dictionary types (IDictionary<TKey, TValue>) rather than generic key/value pair collections, aligning the decoder contract with Slice dictionary semantics (no duplicate keys) and simplifying insertion logic.

Changes:

• Tighten DecodeDictionary / DecodeDictionaryWithOptionalValueType generic constraints from ICollection<KeyValuePair<...>> to IDictionary<...>.
• Switch decoder insertion from Add(KeyValuePair<...>) to Add(key, value) for both Slice and Ice decoders.
• Remove generator tests and Slice definitions that relied on List<KeyValuePair<...>> as a dictionary mapping target.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tests/IceRpc.Slice.Generator.Tests/DictionaryMappingTests.slice	Updates dictionary mapping expectations to IDictionary and removes List<KeyValuePair>-based operations.
tests/IceRpc.Slice.Generator.Tests/DictionaryMappingTests.cs	Removes tests that exercised List<KeyValuePair> dictionary mappings.
src/ZeroC.Slice.Codec/SliceDecoderExtensions.cs	Tightens dictionary decode constraints to IDictionary and uses Add(key, value) (including optional-value variant).
src/IceRpc/Ice/Codec/IceDecoderExtensions.cs	Tightens dictionary decode constraint to IDictionary and uses Add(key, value).

[bernardnormier]

Don't forget to update the Slice doc after merging this PR.

[InsertCreativityHere]

Looks good to me!