fix: inline SDK version instead of reading package.json at runtime

[Zaimwa9]

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Changes

getUserAgent() read the SDK version with require('../package.json') at request time. This never worked in the published package (the relative path resolves to build/esm/package.json or the build/cjs/package.json stub, so the header always reported unknown), and under bundlers such as Vite the require can be rewritten into a form that escapes the try/catch and crashes the host application.

• Inline the version in a committed sdk/version.ts constant; no filesystem or require access at runtime.
• Register sdk/version.ts as a release-please extra-files entry so the constant is bumped on every release.
• Un-skip the ESM user-agent test, which now passes against the real package version.

How did you test this code?

• npm test and npm run test:esm-build pass.
• Built and packed the SDK, bundled a Fastify server with vite build --ssr (Vite 8), ran the bundle from a separate directory: the request sends User-Agent: flagsmith-nodejs-sdk/9.0.1 and the bundle contains no package.json reference.