Add cross-version interop test against the published ext-apps 1.7.5

[ochafik]

Adds a unit test that runs the published ext-apps 1.7.5 against this build over an in-memory transport, in both pairings: 2.x App with the 1.x AppBridge (outer client on sdk 1.30), and 1.x App with the 2.x AppBridge (outer client on SDK 2.0). It complements src/wire-compat.test.ts, which replays hand-shaped 1.x JSON: that file pins message shapes, this one runs the real 1.x code.

Why it earns its place: mutation-testing showed a coordinated wire rename in 2.x (sender and shared schema literal changed together, e.g. ui/notifications/tool-cancelled, ui/open-link's url param, or ui/initialize's appInfo) is invisible to the whole same-version suite and to wire-compat.test.ts, and is caught only here. It also carries an exact assertion on the raw ui/initialize result the 1.x View receives, so additive drift fails too.

Mechanics: two devDependencies (ext-apps-v1 as an npm alias of @modelcontextprotocol/ext-apps@1.7.5, and @modelcontextprotocol/sdk@1.30.0, whose name does not collide with the 2.x split packages; a second alias for the sdk would install it twice and break its instanceof McpError checks), and one test file. A provenance test asserts the alias resolves to 1.7.5 and that the 1.x classes descend from sdk-1's Protocol while the 2.x ones do not, so the pairing cannot silently collapse to same-version. 15 tests, 57 assertions.

Checked: npm audit unchanged versus main; lockfile entries all registry.npmjs.org; check-versions, check-dependency-isolation, npm pack contents, prettier and tsc --noEmit unaffected; the test step runs offline in the CI image (--network none) after npm ci. 10 of 10 native runs green. The cases in each pairing share one connected fixture and run in file order, noted in a comment.

The 1.7.5 pin is deliberate: it is the last 1.x, and bump-version.mjs leaves it alone. A deliberate 2.x wire change would need a pairings-table decision here, which is the point.

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commit: 5f6346e