fix(nginx): use text/csv in gzip_types instead of test/csv

[gu-feng418]

What this does

gzip_types in the agent, origin and proxy nginx templates listed test/csv, which is not a MIME type nginx knows, so the entry never matched any response and CSV bodies were served uncompressed. This changes all three templates to text/csv.

gzip_types text/plain test/csv application/json;   ->   gzip_types text/plain text/csv application/json;

Affected files: nginx/config/agent.go, nginx/config/origin.go, nginx/config/proxy.go. nginx/config/base.go already used the correct types, so it is unchanged.

Why

Fixes #711.

The issue also asks whether CSV compression is wanted at all. I kept the entry and corrected the type, since listing it next to text/plain and application/json shows the original intent; text/csv is the registered type for that body. If you would rather not gzip CSV, dropping the token from the three templates is a one-line-per-file change instead, and I can update this PR.

Verification

• grep -rn "test/csv" . returns nothing; the only remaining gzip_types lines are the corrected ones plus the unrelated list in base.go.
• go build ./nginx/..., go vet ./nginx/config pass and gofmt -l ./nginx/ is clean.
• The strings are plain template constants with no unit tests in nginx/config, so no test changes were needed.

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