refactor: decompose createHandler in fastify.ts into focused helper functions

[Copilot]

createHandler was a ~480-line monolithic function with significant duplication across route registration, error response serialization, and plugin execution patterns.

Structural decomposition

• Extract 14 focused module-level functions (registerPreHandler, registerErrorHandler, createRouteHelpers, createContext, etc.) — createHandler is now ~40 lines that read as a sequential recipe
• All helpers take explicit parameters (no closure captures over createHandler locals), improving testability

DRY patterns

• Route helpers: 8 identical throwOnDefinedRoute(METHOD, path, options); app.method(path, handler) lambdas → single route() factory
• Error responses: 5 duplicate JSON.stringify({ message, code, data }) → toErrorResponseBody()
• Plugin execution: Repeated try/catch-log-rethrow loop → generic runPlugins<T>()
• Headers constant: 4 inline { "Cache-Control": "no-store" } objects → NO_STORE_HEADERS
• Route init: Hand-typed 18-key object literal → ALL_HTTP_METHODS array + createDefinedRoutes()

Control flow cleanup

• throwOnDefinedRoute: replaced confusing if/else if/else if with early-return guards

// Before: confusing negated condition chains
} else if (definedRoutes[type].includes(path) === false) {
    return;
} else if (options?.override === true) {
    return;
}

// After: single clear guard
if (!definedRoutes[type].includes(path) || options?.override === true) {
    return;
}

All 13 existing tests pass unchanged.

Warning

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I tried to connect to the following addresses, but was blocked by firewall rules:

• download.cypress.io
  • Triggering command: /opt/hostedtoolcache/node/24.14.0/x64/bin/node /opt/hostedtoolcache/node/24.14.0/x64/bin/node dist/index.js --exec install (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
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