Refactor acceptance_tests.rb: Add DRY helper methods to reduce duplication

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This PR refactors spec/acceptance/acceptance_tests.rb to make it more DRY, concise, and maintainable while preserving all existing logic and test coverage.

Changes Made

File size reduction: 536 → 452 lines (84 lines removed, ~15.7% reduction)

Added 10 helper methods to eliminate code duplication:

• make_request() - Unified HTTP request handling for GET/POST operations
• expect_response() - Standardized response validation with optional body content checking
• parse_json_response() - Consistent JSON parsing across all tests
• json_headers() - Reusable header construction with Content-Type defaults
• generate_hmac_signature() - Generic HMAC signature generation with configurable algorithms
• generate_hmac_with_timestamp() - HMAC signature generation with timestamp patterns
• generate_slack_signature() - Slack-specific signature format (v0= prefix)
• current_timestamp() - ISO8601 timestamp helper
• unix_timestamp() - Unix timestamp helper
• expired_timestamp() / expired_unix_timestamp() - Test data helpers for expired timestamps

Before/After Comparison

Before (repetitive):

response = http.post("/webhooks/github", payload.to_json, {
  "Content-Type" => "application/json",
  "X-Hub-Signature-256" => "sha256=" + OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new("sha256"), FAKE_HMAC_SECRET, payload.to_json)
})
expect(response).to be_a(Net::HTTPSuccess)
body = JSON.parse(response.body)
expect(body["status"]).to eq("success")

After (DRY):

json_payload = payload.to_json
signature = generate_hmac_signature(json_payload, FAKE_HMAC_SECRET)
headers = json_headers("X-Hub-Signature-256" => signature)
response = make_request(:post, "/webhooks/github", json_payload, headers)
expect_response(response, Net::HTTPSuccess)
body = parse_json_response(response)
expect(body["status"]).to eq("success")

Benefits

• Eliminated repetitive code - No more duplicate HTTP request setup, header construction, or response validation
• Standardized patterns - Consistent approach to HMAC signature generation and response handling
• Improved readability - Tests focus on business logic rather than boilerplate
• Easier maintenance - Changes to common patterns can be made in one place
• Preserved functionality - All existing test logic maintained with 100% test coverage

Testing

• ✅ All unit tests pass (502 examples, 0 failures)
• ✅ All integration tests pass (8 examples, 0 failures)
• ✅ 100% test coverage maintained
• ✅ Linting passes with no offenses
• ✅ No behavioral changes to any tests

Fixes #39.

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Pull Request Overview

This PR refactors spec/acceptance/acceptance_tests.rb by extracting common logic into helper methods to reduce duplication and improve maintainability, and bumps the library version.

• Introduced DRY helper methods for request handling, JSON parsing, and signature generation.
• Reduced file size by ~15.7% while preserving all existing test logic and coverage.
• Updated VERSION constant to 0.0.4 in lib/hooks/version.rb.