Handle Server Actions on dynamic PPR fallback routes (#96932)
## Summary
A fetch action can be dispatched to a parameterized route fallback
without concrete route params. When the deployment adapter also supplies
that route’s postponed state, Next.js throws because postponed state and
fallback params are present together.
These requests only need to execute the action and cannot safely render
the fallback route. This change preserves the Resume Data Cache for
cached reads, discards the React postponed state that cannot be resumed
without concrete params, and consistently skips fallback-route rendering
for successful and failed actions.
## Verification
- Added a `next start` integration test that synthesizes an action-only
fallback request and fails without the fix.
- Added coverage for access fallback errors and direct Resume Data Cache
extraction.
- Added a deployment test using a public browser interaction where an
action is retained across navigation to a route that does not bundle it.
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