Refactor serial command handling, add HTTP handlers, and expand parser tests

[bootjp]

Motivation

• Reduce duplication and clarify serial-command flow by extracting helpers and named constants for commands and header length.
• Make the parsing logic more testable and robust by isolating key/value parsing into a dedicated function.
• Separate HTTP handler wiring from run to improve readability and make the HTTP endpoints easier to adjust.
• Increase unit-test coverage for the parser to validate edge cases and input robustness.

Description

• Introduce constants startCommand, stopCommand, and commandHeaderLines and use them where commands/header skipping are needed in main.go.
• Extract parseKeyValue, openSerialPort, and writeCommand helper functions and update recordMetrics to use these helpers and a loop-driven header-skip instead of inline logic.
• Add metricsHandler and indexHandler functions and register them in run instead of inlining the handler code.
• Expand main_test.go with named subtests and additional cases (invalid values, unknown keys ignored, missing equals ignored) to exercise parser behavior.

Testing

• Ran go test ./... and all tests passed.
• Updated unit tests (main_test.go) and executed them as part of the test run which succeeded.

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Codex Task

[github-actions]

🚫 [golangci] _{reported by reviewdog 🐶}
error returned from external package is unwrapped: sig: func strconv.ParseFloat(s string, bitSize int) (float64, error) (wrapcheck)

[github-actions]

🚫 [golangci] _{reported by reviewdog 🐶}
error returned from external package is unwrapped: sig: func strconv.ParseFloat(s string, bitSize int) (float64, error) (wrapcheck)

[github-actions]

🚫 [golangci] _{reported by reviewdog 🐶}
error returned from external package is unwrapped: sig: func strconv.ParseFloat(s string, bitSize int) (float64, error) (wrapcheck)