fix: add non-JSON response handling in HttpProvider and empty content validation

[knqiufan]

Summary

• Bug [Enhancement]: TypeScript 完整复刻 PowerMem #4 — HttpProvider non-JSON response handling: HttpProvider.request() called res.json() without safety checks; when the server returned HTML (e.g. 502 Bad Gateway) or plain text, callers received a raw SyntaxError instead of a meaningful API error. Now checks Content-Type header first, catches JSON parse failures, and throws PowerMemAPIError with status code and truncated response body (max 200 chars). Matches Python pattern in rerank/generic.py (try json() → except ValueError → response.text).
• Bug refactor: restructure to match Python powermem + full feature replication #5 — Empty content validation: add({content: ''}) and update(id, {content: ' '}) silently generated empty memories, wasting storage and embedding API calls. Now validates at multiple layers (NativeProvider + Memory facade) using !content || !content.trim(), matching Python's defensive not content or not content.strip() guards in memory.py. Also replaces throw new Error with throw new PowerMemError('...', 'NOT_FOUND') in update().

Changed files

File	Change
src/core/http-provider.ts	Add Content-Type check, JSON parse try-catch, meaningful error messages
src/core/native-provider.ts	Add validateContent(), validate in add()/update(), use PowerMemError
src/core/memory.ts	Add fast-fail empty content validation in facade add()/update()
tests/unit/core/http-provider.test.ts	New: 9 test cases for JSON/non-JSON/connection error handling
tests/regression/edge-cases.test.ts	Add 7 empty content validation test cases

Test plan

• All 355 tests pass across 33 test files (0 failures)
• HttpProvider tests cover: valid JSON, HTML response, plain text, missing Content-Type, malformed JSON, success=false, connection error, 404 handling, body truncation
• Empty content tests cover: empty string add, whitespace-only add, empty string update, whitespace-only update, metadata-only update (no false positive), batch with empty item, NOT_FOUND error code