Simplify ConnectionConfig — remove url and proxyConnection

## Parent PRD

- Parent #1618

## What to build

Remove `url` and `proxyConnection` from `ConnectionConfig` and `NormalizedConnection` types. Make `graphDbUrl` required. Simplify `normalizeConnection` to remove the proxy inference logic. Update `fetchDatabaseRequest`/`getAuthHeaders` to always send proxy headers (`graph-db-connection-url`, `db-query-logging-enabled`) unconditionally.

This is the core type change that cascades through the codebase, eliminating conditional branches around proxy vs direct connections.

**Type: HITL** — requires review before completion.

## Acceptance criteria

- [ ] `ConnectionConfig` no longer has `url` or `proxyConnection` fields
- [ ] `graphDbUrl` is required on `ConnectionConfig`
- [ ] `NormalizedConnection` reflects the simplified type
- [ ] `normalizeConnection` no longer infers `proxyConnection`
- [ ] `getAuthHeaders` always sends `graph-db-connection-url` and `db-query-logging-enabled` headers
- [ ] All type errors resolved across the codebase
- [ ] Tests for `normalizeConnection` updated
- [ ] Tests for `fetchDatabaseRequest`/`getAuthHeaders` verify headers are always sent

## Blocked by

- Blocked by #1621

## User stories addressed

- User story 10
- User story 15