Motivation:

Swift 6.1 has been released, we should add it to our CI coverage.

Modifications:

Add additional Swift 6.1 jobs where appropriate in main.yml,
pull_request.yml

Result:

Improved test coverage.

Move nightly disable parameter in main.yml - the matrix workflow doesn't
accept this parameter, it's in the wrong place. It's already in the
correct place in the pull request yaml.

Add /swift-openapi-runtime as safe directory. This is already set in the
pull request yaml but is missing here

### Motivation

Don't have typos in error messages.

### Modifications

Fixed the typo.

### Result

Typo no more.

### Test Plan

N/A

…dings in anyOf/allOf (#156)

### Motivation

Fixes apple/swift-openapi-generator#808.

More background: We used to use `singleValueContainer()`, provided by
Codable, to encode not just primitive values, but also composite values
like objects and arrays. This works in _most_ cases, but not when you
have an `anyOf` (or an `allOf`), which contains more than 1 of these
types (`OpenAPIValueContainer` or `OpenAPIObjectContainer`). When it
hits that case, it crashes at runtime, because you can't encode multiple
composite types into a single encoder using `singleValueContainer()`.

However, you can do that when using the proper `container(keyedBy:)` and
`unkeyedContainer()` APIs, as that way you're not overwriting the value
that's already there, but amending it.

### Modifications

Improve the encoding and decoding logic of `OpenAPIValueContainer` and
`OpenAPIObjectContainer` to use the more appropriate Codable APIs when
coding composite types, like objects and arrays.

### Result

Now we can have an anyOf/allOf with multiple of these types, and they
encode/decode correctly. I also verified that the original example
reported by the user works with this patch.

### Test Plan

Added more unit tests and ensured all still pass, also verified that
Swift OpenAPI Generator's tests pass when pointed at this patched
version of Runtime.