fix(client): keep a param value of "index" in $url() and $path() - #5297

Summary

removeIndexString() strips a trailing /index so the synthetic .index
accessor resolves to the route it aliases. In the $url() / $path() branch it
runs after the path params have been substituted, so anything that merely
looks like the alias once substitution is done gets stripped as well.

Moving the call ahead of substitution fixes it: the alias is part of the route
template, and a param value is not.

Before / after

The third row is a route with a literal /index segment rather than a param
value. Dispatching against that route alone, / returns 404 and /index
returns 200, so the generated URL did not point at the route it came from.

Why this ordering

ClientRequestImpl.fetch has done removeIndexString → replaceUrlParam since
hc was introduced in #862, which is why $get() already returns the right path
for all three rows above and only $url() / $path() disagreed.

This is a follow-up to #4267, which added the removeIndexString call to this
branch (fixing #4265) and placed it at the end of the sequence, immediately
before return new URL(result). Moving it to the front brings the branch in line
with fetch.

removeIndexString's (?=\?|$) lookahead, added in #4352, is left as-is: it is
still exercised directly by src/client/utils.test.ts, even though no hc call
site now passes it a query string.

Relationship to #5291

#5291 adds the same normalization to the $ws() branch. The two are independent
hunks in the same function and complementary. I merged them locally in a scratch
branch to check: no conflict, and src/client/client.test.ts passes 128 with
both applied. With both in, all four accessors ($get(), $url(), $path(),
$ws()) strip the alias from the route template before substituting params.
Happy to rebase on top of #5291 if it lands first.

Tests

5 tests added, 9 executions — four of them sit in the $path / $url
describe.each, and one in the query-option block. 7 fail on main. The one
that passes either way — Should still drop the index alias of a route with a param — is a regression guard that the genuine alias is still removed.

Verification

• npx vitest --run src/client/client.test.ts — 127 passed (120 with client.ts
  reverted to main, 7 failed)
• npx tsc -p tsconfig.spec.json — exit 0
• npx eslint src — exit 0
• Prettier clean on the committed blobs
• Full npx vitest --run: green with --coverage.enabled=false. With coverage
  on I see at least one pre-existing 5 s timeout,
  src/middleware/compress/index.test.ts "should compress streaming responses
  written in multiple chunks", which reproduces identically on a clean main
  checkout and passes 45/45 in isolation.

The author should do the following, if applicable

• Add tests
• Run tests
• bun run format:fix && bun run lint:fix to format the code (verified with prettier --check and eslint directly; the fixer itself rewrites every line ending on a CRLF checkout)
• Add TSDoc/JSDoc to document the code