fix(mcp): validate search_graph inputs before Zep API calls

[vedikatai]

Summary

Adds defensive input validation to the Go MCP server search_graph handler so malformed or abusive tool arguments are rejected before they reach the Zep API.

This closes a validation gap where SearchGraphInput fields were passed through with no bounds checking, empty-string checks, or max-length enforcement.

Security rationale

The MCP server is an untrusted boundary: clients (agents, IDEs, scripts) can send arbitrary tool arguments. Without local validation:

1. Empty / whitespace-only queries waste API quota and can surface confusing downstream errors.
2. Oversized queries (>2000 characters) increase request payload size and can stress embedding/search backends.
3. Out-of-range limit (negative or very large) can cause unintended expensive scans or invalid API calls.
4. Out-of-range min_fact_rating (the score threshold field on this tool) can send nonsensical filter values to the API.

Failing fast with clear fmt.Errorf messages matches existing handler/transform error patterns and keeps bad inputs out of the network path.

Changes

SearchGraphInput.Validate() — mcp/zep-mcp-server/internal/handlers/types.go

New method enforces:

Rule	Behavior
Empty / whitespace-only query	rejected (query cannot be empty)
query > 2000 runes	rejected (query exceeds maximum length of 2000 characters)
Explicit limit outside 1–100	rejected (limit must be between 1 and 100); 0 remains the "use default" sentinel
min_fact_rating outside [0.0, 1.0]	rejected (min_fact_rating must be between 0 and 1); 0 still means omitted

Constants: searchQueryMaxLen, searchLimitMin/Max, searchMinScoreMin/Max (lines 10–16 in types.go).

Error style mirrors internal/transform.ValidateRequired (fmt.Errorf("... cannot be empty"), etc.).

Handler wiring — mcp/zep-mcp-server/internal/handlers/search.go

HandleSearchGraph now calls input.Validate() before applying defaults and building zep.GraphSearchQuery (lines 17–19), so explicit out-of-range limits are rejected rather than being overwritten by defaults.

Graph handler (graph.go) check

There is no internal/handlers/graph.go in this package. Graph search is implemented only in:

• search.go — HandleSearchGraph (the tool fixed here)
• Read-only graph entity handlers (nodes.go, edges.go, episodes.go, node_detail.go, edge_detail.go, episode_detail.go, node_edges.go, episode_mentions.go) which take UUIDs/IDs/limits only and do not accept free-text search queries or score thresholds

So the validation gap was specific to search_graph, not a missing parallel graph.go file.

Tests added

All in existing mcp/zep-mcp-server/internal/handlers/types_test.go via new TestSearchGraphInputValidate (table-driven, same style as TestGetUserInputValidation):

Valid

• valid minimal query
• valid query with surrounding whitespace
• valid query at max length (2000 chars)
• valid limit omitted (zero uses handler default)
• valid limit at minimum boundary (1)
• valid limit at maximum boundary (100)
• valid min_fact_rating omitted (zero)
• valid min_fact_rating at minimum boundary (0.0)
• valid min_fact_rating at maximum boundary (1.0)
• valid min_fact_rating mid range (0.5)
• limit zero is default sentinel not error

Invalid

• empty query
• whitespace only query
• query exceeds max length (2001 chars)
• limit below minimum (-1)
• limit above maximum (101)
• min_fact_rating below minimum (-0.01)
• min_fact_rating above maximum (1.01)

Verification

cd mcp/zep-mcp-server && go test ./...

All packages pass locally.

Test plan

• go test ./... under mcp/zep-mcp-server passes
• Empty/whitespace query rejected before API call
• Query length boundary at 2000 / 2001
• Limit boundaries: 0 (default ok), 1, 100 ok; -1, 101 rejected
• min_fact_rating boundaries: 0, 1 ok; -0.01, 1.01 rejected
• CI green on this PR