Add "Limit" QueryFilter to Etre queries

[jemiahw]

We can now pass a limit in QueryFilter (with --limit N in the CLI) to limit the number of rows returned. This works for distinct and non-distinct queries.

[Copilot]

Pull request overview

This PR adds a Limit field to the etre.QueryFilter struct, allowing callers to cap the number of entities returned by a query. The limit is supported in both the CLI (--limit N) and the Go client, wires through the API server (validated as a non-negative integer), and is enforced in the data store for both regular and distinct queries.

Changes:

• etre.go / entity_client.go: Add Limit int64 to QueryFilter; client serializes it as a &limit=N query parameter when positive.
• api/api_gomux.go / es/es.go / es/config/config.go: Server-side parsing and validation of the limit query param; CLI flag --limit with input validation.
• entity/store.go: Applies limit via MongoDB's SetLimit for regular queries and via post-fetch slicing for distinct queries (since MongoDB's distinct command doesn't natively support a limit).

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
etre.go	Adds Limit int64 field to QueryFilter with a doc comment
entity_client.go	Serializes Limit > 0 as &limit=N in query URL; imports strconv
api/api_gomux.go	Parses and validates limit query parameter; rejects negative or non-numeric values with HTTP 400
entity/store.go	Applies limit via SetLimit for cursor-based queries; slices result for distinct queries
es/es.go	Validates --limit is non-negative before query; passes it through QueryFilter
es/config/config.go	Adds Limit int64 option field and help text
client_test.go	Tests that Limit > 0 is sent as query param and Limit == 0 is not
api/query_test.go	Tests parsing of valid, zero, negative, and non-numeric limit values in the API handler
entity/store_test.go	Tests limit behavior in StreamEntities for various combinations (zero, positive, over-limit, with labels, distinct)

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