fix(schema): emit unique_identifiers for @upsert/@lookup nodes

[shawkatkabbara]

Problem

build_schema_params() was producing node_types without the unique_identifiers field, even though SchemaCreateParams documents it as:

Equivalent to @upsert/@lookup decorators... Properties marked as unique_identifiers are used for entity deduplication and merging.

Without this field, the memory server's _get_unique_identifiers_for_node_type() returns [] and falls back to _create_node_with_content_check, creating duplicate nodes for the same logical entity.

Reproduction

Register a schema with @upsert + prop(required=True, search=exact()):

@node
@upsert
class call:
    call_id: str = prop(required=True, search=exact())
    name: str = prop(required=False, search=semantic())

Before this PR:

{
  "name": "call",
  "properties": {...},
  "unique_identifiers": []   // ❌ empty
}

After this PR:

{
  "name": "call",
  "properties": {...},
  "unique_identifiers": ["call_id"]   // ✅ populated
}

Real-world impact

In production data we observed 11 duplicate :call nodes sharing identical call_id values, caused entirely by this gap. After running with the fix, only one node is created per call_id (server-side MERGE on unique_identifiers succeeds).

Fix

Derive unique_identifiers from properties marked required=True + search=exact() (matching the documented contract) and emit on @upsert and @lookup nodes.

Tests

Locally verified:

• @upsert call with call_id: str = prop(required=True, search=exact()) → unique_identifiers: ["call_id"] ✓
• @lookup tactic with id: str = prop(required=True, search=exact()) → unique_identifiers: ["id"] ✓
• Nodes with no exact-search required properties → field omitted (no behavior change) ✓
• @create (default) policy → field omitted (only emitted on upsert/lookup) ✓