Concatenate adjacent string literals in deprecated attribute messages

[tannergooding]

Fixes #694.

A C deprecated message can be written as adjacent string literals, which the compiler concatenates into a single value:

[[deprecated("Use Bar() or "
             "Baz() instead")]]
void MyFunction();

The generator extracted the message as the substring between the first " (via IndexOf) and the last " (via LastIndexOf) and emitted it verbatim. For adjacent literals that span text includes the interior boundary quotes and inter-literal whitespace/newlines, so it emitted invalid C#:

[Obsolete("Use Bar() or "
    "Baz() instead")]

The fix replaces the fragile first/last-quote slice with a proper scan (GetDeprecatedMessage) that walks the attribute source, gathers the contents of every string literal (respecting escaped quotes so \" isn't treated as a terminator), and joins them into a single value:

[Obsolete("Use Bar() or Baz() instead")]

The common single-literal path is byte-identical (verified: no golden output changed), escaped quotes are preserved, and a missing/empty message still emits a bare [Obsolete].

Added DeprecatedAdjacentStringTest covering the adjacent-literal case. Build 0 warning / 0 error (Release, TreatWarningsAsErrors + EnforceCodeStyleInBuild); full suite green (3713 passed / 0 failed).

Note

This PR description and the code change were drafted by Copilot on my behalf.