Fix source field typespecs in HTTPError and TransportError

[cgarvis]

The source field in Finch.HTTPError and Finch.TransportError was typed as Mint.HTTPError.t() | nil and Mint.TransportError.t() | nil respectively. The set-theoretic type checker surfaced by elixir-lang/elixir#15366 treats @type t() definitions as closed-map types. Because Mint.HTTPError.t() only declares {reason: reason()} and omits the :module field present in the actual struct, a value matched with %Mint.HTTPError{reason: reason, module: module} = error carries a :module field that falls outside the declared type. This produces a :badstructfield diagnostic for the :source field at the construction site.

The :source field holds the original Mint exception so that Exception.message/1 can delegate to it. Exception.t() is the accurate type for that usage: it is an open-map type that admits any exception struct, including both Mint.HTTPError and Mint.TransportError.

Before:

# http_error.ex
@type t() :: %__MODULE__{
        reason: reason(),
        module: module() | nil,
        source: Mint.HTTPError.t() | nil
      }

# transport_error.ex
@type t() :: %__MODULE__{reason: reason(), source: Mint.TransportError.t() | nil}

After:

# http_error.ex
@type t() :: %__MODULE__{
        reason: reason(),
        module: module() | nil,
        source: Exception.t() | nil
      }

# transport_error.ex
@type t() :: %__MODULE__{reason: reason(), source: Exception.t() | nil}

mix test: 206 tests, 0 failures, 1 skipped.