make uri serializer more tolerant to type errors

[beyang]

Make the URI deserializer tolerant to the following errors that can occur in deserialization:

• value is null or missing
• value is a string, rather than a URI object

Previously, we saw the following errors:

Example 1: #526
Example 2:

2024-02-08 18:20:43,368 [  32696] SEVERE - o.e.l.j.j.StreamMessageProducer - An error occurred while processing an incoming message.
java.lang.NullPointerException
   at com.sourcegraph.cody.agent.protocol.ContextFileKt.uriDeserializer$lambda$1(ContextFile.kt:42)
   at com.google.gson.internal.bind.TreeTypeAdapter.read(TreeTypeAdapter.java:76)
   at com.google.gson.Gson.fromJson(Gson.java:1227)
   at com.google.gson.Gson.fromJson(Gson.java:1329)
   at com.google.gson.Gson.fromJson(Gson.java:1300)

Note: this PR also includes changes from #555

Test plan

Tested locally

[pkukielka]

Thanks! LGTM