fix: add root src/index.ts so the advertised root export resolves (#2273)

[ousamabenyounes]

Summary

Fixes #2273

package.json (v1.x) advertises a root . export:

"exports": { ".": {
  "types": "./dist/esm/index.d.ts",
  "import": "./dist/esm/index.js",
  "require": "./dist/cjs/index.js"
} }

but there was no src/index.ts, so tsc never emitted those files and they were absent from the published tarball. Both an ESM import and a CJS require of the package root threw ERR_MODULE_NOT_FOUND before any consumer code ran. Subpath exports (/client, /server, …) were unaffected.

This adds a side-effect-free root barrel per the direction in the issue triage:

• export * from './types.js' — protocol types, schemas, constants
• export * from './inMemory.js' — InMemoryTransport

Client and Server intentionally stay on their ./client / ./server subpath exports to avoid TS2308 ambiguous re-export collisions. The change is additive and non-breaking; main is unaffected (it is a monorepo with no root package).

Test verification (RED → GREEN)

New regression test test/issues/test_2273_root_export.test.ts imports the root barrel and asserts it resolves and re-exports the protocol surface + a working InMemoryTransport.

RED — new test on unmodified v1.x (no src/index.ts):

FAIL  test/issues/test_2273_root_export.test.ts
Error: Cannot find module '../../src/index.js' imported from '.../test/issues/test_2273_root_export.test.ts'
 Test Files  1 failed (1)

GREEN — with src/index.ts added:

✓ test/issues/test_2273_root_export.test.ts (3 tests) 7ms
 Test Files  1 passed (1)
      Tests  3 passed (3)

End-to-end packaging check — after npm run build, the previously-missing root files now emit and both loaders resolve:

dist/esm/index.js EXISTS
dist/cjs/index.js EXISTS
dist/esm/index.d.ts EXISTS
ESM OK — symbols: 172 — LATEST: 2025-11-25 — InMemoryTransport: function
CJS OK — symbols: 172 — JSONRPC_VERSION: 2.0
npm pack --dry-run: all three root files present in the tarball

Full local suite

• npm run typecheck (tsgo), npm run lint (eslint + prettier), npm run build, npm test (vitest): all green.
• No regressions: baseline 52 files / 1639 tests → with fix 53 files / 1642 tests (+1 file, +3 new tests, 0 failures).

Files changed

File	Change
src/index.ts	New side-effect-free root barrel re-exporting ./types.js + ./inMemory.js
test/issues/test_2273_root_export.test.ts	Regression test for the root export
.changeset/fix-root-export-missing-index.md	Changeset (sdk package, patch)

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