fix(fastify): append headers instead of overwriting

[bun-unsafe]

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
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PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

AbstractHttpAdapter.appendHeader / ExpressAdapter.appendHeader append a header value (response.append). FastifyAdapter.appendHeader calls Fastify reply.header(), which replaces any existing value except for set-cookie.

Calling appendHeader twice with the same name (for example x-a: 1 then x-a: 2) therefore keeps only the last value on Fastify, while Express keeps both.

This dates back to the original adapter methods in #12955.

Issue Number: N/A

What is the new behavior?

FastifyAdapter.appendHeader concatenates onto the current header when one is already set, matching Express.

set-cookie still goes through Fastify reply.header() only. Fastify already concatenates that name; passing the accumulated list back into header() would duplicate previous cookies.

Does this PR introduce a breaking change?

• Yes
• No

Call sites that used appendHeader as a second setHeader on Fastify would start seeing multiple values. That matches the method name and ExpressAdapter.

Other information

Repro: Fastify adapter, appendHeader(res, 'x-a', '1') then appendHeader(res, 'x-a', '2'), then getHeader(res, 'x-a'). Before: '2'. After: ['1', '2'].

Tests: packages/platform-fastify/test/adapters/fastify-adapter.spec.ts (append twice, setHeader then append, header name case, three values, set-cookie without duplicates).

npx vitest run packages/platform-fastify/test/adapters/fastify-adapter.spec.ts