Fix metadata prefetch cache key for search params (#97729)
### What?
Treat a `LoadingBoundary` response as `Full` when writing the response's
head into the segment cache, matching the strategy already used when the
scheduler creates that head entry.
Adds a production e2e regression test with two concurrent prefetches for
the same pathname and different search params.
### Why?
`pingRuntimeHead` creates loading-boundary head entries with the
concrete `Full` strategy because a head has no loading boundary. The
canonical re-keying added in #96095 later wrote the fulfilled head with
raw `LoadingBoundary` semantics, which drops search params from the
metadata cache key. Concurrent query-specific metadata responses could
therefore replace each other under one fallback key; whichever response
settled last supplied the title for both URLs.
Fixes #97657
### How?
Normalize `LoadingBoundary` to `Full` at the head write boundary only.
Page segments retain loading-boundary keying, and shell/PPR responses
keep their existing strategies.
The test reveals alpha and beta links in one React render, waits for
both prefetches, navigates to alpha, and verifies both body and title.
It then performs a search-only navigation to an unprefetched gamma URL
and verifies that the head is replaced.
### Testing
- Regression boundary: test passes at `e70495c1ee` and fails at #96095
merge commit `dfa7f4f713` without the fix (`alpha` body, `beta` title).
- Test passes at `dfa7f4f713` with the fix.
- `NEXT_SKIP_ISOLATE=1 pnpm test-start-turbo
test/e2e/app-dir/segment-cache/metadata-search-params/metadata-search-params.test.ts`
(pass, plus 3 consecutive repeat runs)
- `NEXT_SKIP_ISOLATE=1 pnpm test-start-webpack
test/e2e/app-dir/segment-cache/metadata-search-params/metadata-search-params.test.ts`
(pass)
- Relevant existing Turbopack suites: 16 tests pass across metadata,
static shell, metadata soft navigation, and stale search-param coverage.
- Existing search-param suites: 6 tests pass with webpack. Their
Turbopack builds are currently blocked on canary by an unrelated Edge
Runtime `process.cwd()` error.
- `pnpm --filter next build`
- Targeted ESLint and Prettier checks
- Structured external review: 0 findings
