chore: add lint configuration and fix lint issue

[pujidjayanto]

• add .golangci.yml as same as https://github.com/stacklok/toolhive/blob/main/.golangci.yml with little adjustment
• fix lint error

test seems okay

task: [test] go test -v ./...
=== RUN   TestCreateServer
--- PASS: TestCreateServer (0.00s)
PASS
ok  	github.com/StacklokLabs/osv-mcp/cmd/server	0.635s
=== RUN   TestHandleQueryVulnerability
--- PASS: TestHandleQueryVulnerability (0.00s)
=== RUN   TestHandleQueryVulnerabilityWithPURL
--- PASS: TestHandleQueryVulnerabilityWithPURL (0.00s)
=== RUN   TestHandleQueryVulnerabilityWithCommit
--- PASS: TestHandleQueryVulnerabilityWithCommit (0.00s)
=== RUN   TestHandleQueryVulnerabilitiesBatch
--- PASS: TestHandleQueryVulnerabilitiesBatch (0.00s)
=== RUN   TestHandleGetVulnerability
--- PASS: TestHandleGetVulnerability (0.00s)
PASS
ok  	github.com/StacklokLabs/osv-mcp/pkg/mcp	1.473s
=== RUN   TestQuery
--- PASS: TestQuery (0.00s)
=== RUN   TestQueryBatch
--- PASS: TestQueryBatch (0.00s)
=== RUN   TestGetVulnerability
--- PASS: TestGetVulnerability (0.00s)
PASS
ok  	github.com/StacklokLabs/osv-mcp/pkg/osv	1.065s

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