## Description

Fixes #5620

Replaces the unsafe `(T)(object)stream` cast pattern with safe `is`
pattern matching in all `FromStream<T>` serializer implementations
across the SDK.

### Problem

The `FromStream<T>` method in multiple serializer implementations uses
the following pattern:

```csharp
if (typeof(Stream).IsAssignableFrom(typeof(T)))
{
    return (T)(object)stream;
}
```

`typeof(Stream).IsAssignableFrom(typeof(T))` returns `true` when `T` is
`Stream` **or any subclass** (e.g., `MemoryStream`, `FileStream`). If
`T` is a specific `Stream` subclass but the runtime `stream` parameter
is a different `Stream` type, the cast `(T)(object)stream` throws a raw
`InvalidCastException` with no context about what went wrong.

**Example that throws:**
```csharp
// T = MemoryStream, but stream is actually a FileStream at runtime
serializer.FromStream<MemoryStream>(someFileStream); // InvalidCastException!
```

### Fix

Replaced the unsafe cast with safe `is` pattern matching:

```csharp
if (typeof(Stream).IsAssignableFrom(typeof(T)))
{
    if (stream is T typedStream)
    {
        return typedStream;
    }

    throw new InvalidCastException(
        $"Stream of type '{stream.GetType().FullName}' is not compatible "
        + $"with the requested type '{typeof(T).FullName}'.");
}
```

This provides:
- ✅ Safe runtime type checking (no unexpected `InvalidCastException`)
- ✅ A descriptive error message identifying both the actual and expected
types
- ✅ No behavioral change for the common case (`T = Stream`)

### Files Changed

**Core SDK Serializers (2):**
- `Microsoft.Azure.Cosmos/src/Serializer/CosmosJsonDotNetSerializer.cs`
-
`Microsoft.Azure.Cosmos/src/Serializer/CosmosSystemTextJsonSerializer.cs`

**Sample Code (3):**
-
`Microsoft.Azure.Cosmos.Samples/Usage/SystemTextJson/CosmosSystemTextJsonSerializer.cs`
-
`Microsoft.Azure.Cosmos.Samples/Usage/ReEncryption/ReEncryptionSupport/ReEncryptionJsonSerializer.cs`
- `Microsoft.Azure.Cosmos.Samples/Usage/ItemManagement/Program.cs`

**Encryption Modules (2):**
- `Microsoft.Azure.Cosmos.Encryption/src/CosmosJsonDotNetSerializer.cs`
-
`Microsoft.Azure.Cosmos.Encryption.Custom/src/Common/CosmosJsonDotNetSerializer.cs`

**Unit Tests (2):**
-
`Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.Tests/CosmosJsonSerializerUnitTests.cs`
— 2 new tests
-
`Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.Tests/Json/CosmosSystemTextJsonSerializerTest.cs`
— 2 new tests

### Not Changed

- **`PatchOperationCore{T}.cs`** — Has a similar-looking pattern but
casts FROM `T` TO `Stream` (upcast), which always succeeds. No fix
needed.
- **Test utility serializers** — Internal test code, not
customer-facing.

### Testing

- 4 new unit tests added (2 per serializer):
- `ValidateFromStreamWithBaseStreamType` /
`TestFromStreamWithBaseStreamType` — Confirms
`FromStream<Stream>(memoryStream)` succeeds (regression test)
- `ValidateFromStreamWithIncompatibleStreamTypeThrowsDescriptiveError` /
`TestFromStreamWithIncompatibleStreamTypeThrowsDescriptiveError` —
Confirms `FromStream<FileStream>(memoryStream)` throws
`InvalidCastException` with a descriptive message containing both type
names
- All existing tests continue to pass

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>