Include injected routes when checking if renderers are needed in SSR builds

[matthewp]

Changes

• SSR builds with only injected routes (no src/pages/ directory) were failing at runtime with No matching renderer found.
• The renderer plugin's build optimization strips renderers from the SSR bundle when it determines no SSR pages need them. The way it determines this was not accounting for injected routes. This fixes it.

Testing

• Added unit tests for hasNonPrerenderedRoute covering: project pages, prerendered pages, endpoint exclusion, and the new includeExternal option with injected routes.
• Verified end-to-end against HiDeoo's reproduction

Docs

• N/A, bug fix

[codspeed-hq]

Merging this PR will not alter performance

✅ 18 untouched benchmarks

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_{Comparing ssr-renderers-injected-routes-only (fb6850e) with main (4eec0f1)}