fix(ci): upgrade macOS runners from macos-14 to macos-15

The macos-14 SDK has breaking header changes (CFAttributedStringRef,
NSExtensionContext undefined) that cause CGO builds of the systray
dependency to fail.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: claude

.github/workflows/pr-build.yml

@@ -121,13 +121,13 @@ jobs:
             cgo: "1"
             name: mcpproxy-windows-arm64.exe
             archive_format: zip
-          - os: macos-14
+          - os: macos-15
             goos: darwin
             goarch: amd64
             cgo: "1"
             name: mcpproxy-darwin-amd64
             archive_format: tar.gz
-          - os: macos-14
+          - os: macos-15
             goos: darwin
             goarch: arm64
             cgo: "1"

.github/workflows/prerelease.yml

@@ -44,13 +44,13 @@ jobs:
             cgo: "0"
             name: mcpproxy-windows-arm64.exe
             archive_format: zip
-          - os: macos-14
+          - os: macos-15
             goos: darwin
             goarch: amd64
             cgo: "1"
             name: mcpproxy-darwin-amd64
             archive_format: tar.gz
-          - os: macos-14
+          - os: macos-15
             goos: darwin
             goarch: arm64
             cgo: "1"

.github/workflows/release.yml

@@ -214,13 +214,13 @@ jobs:
             cgo: "1"
             name: mcpproxy-windows-arm64.exe
             archive_format: zip
-          - os: macos-14
+          - os: macos-15
             goos: darwin
             goarch: amd64
             cgo: "1"
             name: mcpproxy-darwin-amd64
             archive_format: tar.gz
-          - os: macos-14
+          - os: macos-15
             goos: darwin
             goarch: arm64
             cgo: "1"