fix(index): preserve global option fields when route options are partial

[ranjeetcao]

Calling Object.assign({}, global, processCompressParams(partialRoute)) copies undefined values from the post-processed route object onto the global params, silently nulling out fields like onInvalidRequestPayload, onUnsupportedRequestEncoding, and onUnsupportedEncoding whenever the route configures only one of them.

Fix the merge order: combine the raw user opts with the raw route opts first, then pass the result through processCompressParams / processDecompressParams once so each derived field is computed from a coherent input rather than shallow-merged from already-processed objects.

Adds unit tests covering both decompress hook fallbacks and the compress onUnsupportedEncoding fallback.

Closes #340

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[ranjeetcao]

Closing as a duplicate of #397 (opened May 11, 6 days before mine — I missed it when scanning the issue). Apologies for the noise.
I'll flag one delta on #397 that might be worth picking up: my branch adds a test for the compress-side onUnsupportedEncoding fallback (the regression covered all three of onInvalidRequestPayload, onUnsupportedRequestEncoding, and onUnsupportedEncoding, but #397's test coverage only touches the first two). Happy to send a follow-up test PR if #397 lands as-is. Test is here for reference: https://github.com/fastify/fastify-compress/pull/398/changes#diff-e13c4cd5dbfbf47dd973a06293644d67cd529850c7d6df9ab4ff12563335536cR474