test: expand httpclient error path coverage

[Copilot]

HTTP callback tests only validated happy paths. Error handling for malformed payloads, transport failures, and body read errors was untested.

Changes

• TestErrorPaths: Table-driven tests for invalid protobufs/JSON, malformed URLs, invalid methods
• TestTransportFailures: Connection errors, unreachable endpoints
• TestBodyReadFailures: Simulated connection drops during response read
• TestErrorSemantics: Validates consistent error handling across protobuf and JSON paths

Coverage

Increases from ~80% to 96%. All error branches in Call() and callJSON() now tested, including:

• Payload parsing errors → 400 status
• Request construction errors → 400 status
• Transport/network errors → 500 status
• Response body read errors → 500 status

Example test structure:

tests := []struct {
    name           string
    setup          func() ([]byte, error)
    expectErr      bool
    expectCode     int32
    expectContains string
}{
    {
        name: "Invalid Method in Request",
        setup: func() ([]byte, error) {
            return pb.Marshal(&proto.HTTPClient{
                Method: "INVALID\nMETHOD",
                Url:    "http://localhost",
            })
        },
        expectErr:      true,
        expectCode:     400,
        expectContains: "Unable to create HTTP request",
    },
    // ...
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Expand httpclient callback tests for error-path coverage</issue_title>
<issue_description>## Summary
HTTP callback client tests focus on happy paths and miss several operational failures.

Problem

Invalid proto payloads, request build errors, transport failures, and response read errors are not comprehensively tested.

Scope

• Add table-driven tests for all key error branches
• Use custom round-tripper/readers to simulate network/body failures
• Verify returned status codes and wrapped errors

Acceptance Criteria

• Error branches in Call/callJSON are covered
• Tests verify consistent error semantics
• make tests passes for affected package

References

• pkg/callbacks/httpclient/httpclient.go</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Expand httpclient callback tests for error-path coverage #238

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