Make value_hint optional to allow constant positional arguments (#172)

<!-- Provide a brief summary of your changes -->

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->

This is one attempt to resolve
https://github.com/modelcontextprotocol/registry/issues/169.

Instead of adding a new "constant" argument type, I expanded the
existing positional one and just dropped the `value_hint` requirement. I
am not sure if this has VS Code impact @sandy081, @connor4312.

Also, removed unneeded release_date values from the samples.

## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->

I tested the current server.json samples in the repo against the new
(less strict) schema. Of course they passed (aside from an unrelated bug
in my previous NuGet sample 🤦).

## Breaking Changes
<!-- Will users need to update their code or configurations? -->

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [ ] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [x] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

docs/server-json/schema.json

@@ -196,8 +196,7 @@
         {
           "type": "object",
           "required": [
-            "type",
-            "value_hint"
+            "type"
           ],
           "properties": {
             "type": {
@@ -217,7 +216,19 @@
               "description": "Whether the argument can be repeated multiple times in the command line.",
               "default": false
             }
-          }
+          },
+          "anyOf": [
+            {
+              "required": [
+                "value_hint"
+              ]
+            },
+            {
+              "required": [
+                "value"
+              ]
+            }
+          ]
         }
       ]
     },