### Motivation

Fixes to get closer to building in Swift 6 mode without diagnostics.

### Modifications

- Made two public types Sendable
- s/#file/#filePath in tests

### Result

Builds with fewer warnings when trying Swift 6 mode (will open another
PR for the remaining work).

### Test Plan

Unit tests passed.

### Motivation

Fixing Issues with Building in Swift 6 for Linux

### Modifications

* Fix Issues with Upstream Iterator

### Result

Compatability with Swift 6 on Linux

### Test Plan

TBD

### Motivation

When receiving containers from adopter code, there might be NSNull
values, which represent a nil value.

Previously, this value would not be treated as nil, instead it'd throw
an error as an unrecognized type.

### Modifications

Handle NSNull and treat it as nil.

### Result

You can provide a container with an NSNull nested value and it'll get
encoded correctly.

### Test Plan

Added a unit test.

Improved encoding of NSNumber in OpenAPIValueContainer

### Motivation

When getting CoreFoundation/Foundation types, especially numbers, they automatically bridge to Swift types like Bool, Int, etc. That casting is pretty flexible, and allows e.g. casting a number into a boolean, which isn't desired when encoding into JSON, as `false` and `0` represent very different values.

Previously, we relied on the automatic casting to know how to encode values, however that produced incorrect results in some cases.

### Modifications

Add explicit handling of CF/NS types and try to encode using that new method before falling back to testing for native Swift types. This ensures that the original intention of the creator of the CF/NS types doesn't get lost in encoding.

### Result

Correct encoding into JSON of types produced in the CF/NS world, like JSONSerialization.

### Test Plan

Added unit tests.


Reviewed by: simonjbeaumont

Builds:
     ✔︎ pull request validation (5.10) - Build finished. 
     ✔︎ pull request validation (5.9) - Build finished. 
     ✔︎ pull request validation (5.9.0) - Build finished. 
     ✔︎ pull request validation (api breakage) - Build finished. 
     ✔︎ pull request validation (docc test) - Build finished. 
     ✔︎ pull request validation (integration test) - Build finished. 
     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 

#110