fix(uri): keep a trailing slash on the base URL so proxied sub-paths survive

[OsamaAnsar]

Problem

Closes #296.

When a client is pointed at a libSQL server hosted behind a reverse proxy on a
sub-path, the last path segment is silently dropped from every request:

createClient({ url: "libsql://example.com/some/sub/path" });
// requests go to  https://example.com/some/sub/v2/pipeline
// expected        https://example.com/some/sub/path/v2/pipeline

The reporter's workaround — appending a trailing / to the URL — is the clue to
the cause.

Cause

encodeBaseUrl builds the base URL with no trailing slash on the path
(https://example.com/some/sub/path). The Hrana client then resolves each
endpoint against it:

new URL(endpoint.pipelinePath /* "v2/pipeline" */, baseUrl);

Per the WHATWG URL resolver, a relative reference resolved against a base whose
path does not end in / replaces the base's last segment. So path is
dropped and v2/pipeline takes its place.

Fix

encodeBaseUrl now appends a trailing slash to a non-empty path, so the base URL
is safe for relative resolution.

• "" and "/" are unchanged (the common Turso Cloud case,
  libsql://db-org.turso.io, is unaffected).
• wss: / ws: callers receive the same normalized URL. The Hrana WebSocket
  handshake connects to the URL directly and isn't sensitive to the trailing
  slash, so this is a no-op there; a directory-style base is also the correct
  shape for a proxied WS deployment.

Tests

packages/libsql-client/src/__tests__/uri.test.ts:

• Updated the three encodeBaseUrl() cases that carry a path to expect the
  trailing slash.
• Added a regression test that resolves v2/pipeline and v3-protobuf/pipeline
  against the encoded base and asserts the full sub-path is preserved, plus the
  root case still resolves to /v2/pipeline.

npx jest --runInBand uri config — green (uri 10, config 23). tsc --noEmit
clean for @libsql/core and @libsql/client.