fix(cli): #1719 correct dynamic route segment key parsing and fail fast on multi-segment routes

[jstockdi]

Related Issue

Resolves #1719

Documentation

N/A — no user-facing documentation changes. (The new fail-fast error message for multi-segment routes is self-documenting.)

Summary of Changes

1. getDynamicSegmentsFromRoute (packages/cli/src/lib/url-utils.js): derive the segment key from the bracket in the route (/\[([^\]]+)\]/) instead of an unanchored .replace(extension, ""), so a param name that contains the extension substring (e.g. [json].js, [posts].ts) no longer produces a mangled key. [json].js now builds correctly to /alpha/index.html with params={"json":"alpha"}.
2. Convert every bracket pair to a URL pattern segment (route.replace(/\[([^\]]+)\]/g, ":$1")) so a nested route no longer leaks a literal [..] into the URLPattern.
3. generateGraph (packages/cli/src/lifecycles/graph.js): detect a route with more than one dynamic segment at graph build time and throw a clear, actionable error (Multiple dynamic route segments are not currently supported (found in ...). Use a single [param] per route.) instead of writing a literal [a]/ directory and then failing with an unrelated TypeError: Cannot read properties of undefined (reading 'resources').
4. Trim the now-unused arguments passed to getDynamicSegmentsFromRoute at both call sites.
5. Added regression tests:
   • packages/cli/test/cases/build.default.dynamic-routing-param-name-ext/ — asserts [json].js builds to /alpha/index.html with the correct params and does NOT emit a literal /[json]/ directory.
   • packages/cli/test/cases/build.default.dynamic-routing-multi-segment/ — asserts a nested [a]/[b].js route fails fast with the clear error.

Scope / follow-up

Full support for nested/multi-segment dynamic routes ([category]/[slug]) is intentionally out of scope here — it spans the graph, prerender, bundle, execute-route-module, static-router and adapters, and the single-segment.key data model would need to become multi-key throughout. This PR makes single-segment routes correct and converts the multi-segment case from a baffling crash into a clear limitation. Full nested-param support is a good follow-up enhancement.

The fail-fast check is applied to the page branch only; API routes are exempt, since they already match multiple params at runtime via URLPattern. Verified against the existing API dynamic-route specs (build.default.api, serve.dynamic-routing).

No breaking changes. Existing single-segment dynamic-route / getStaticPaths / SSR behavior is unchanged (verified against the existing dynamic-routing, getStaticPaths, and SSR specs for build, serve, and develop).