Preserve delegate content type for multipart parameters

[seonwooj0810]

Fixes #2813

Problem

When a multipart request contains a POJO/JSON part, DelegateWriter encodes the value through the delegate Encoder (e.g. the Jackson encoder), which correctly sets Content-Type: application/json on the throwaway RequestTemplate. That header was then discarded, and SingleParameterWriter hard-coded Content-Type: text/plain. As a result, JSON parts were emitted as text/plain instead of application/json (see the report in #2813).

Change

• DelegateWriter now reads the Content-Type produced by the delegate and forwards it to a new SingleParameterWriter.writeWithContentType(output, key, value, contentType) method.
• When the delegate sets no content type, the writer falls back to the previous text/plain; charset=<charset> behaviour, so plain single parameters are unaffected.
• The existing write(output, key, value) template method is preserved and simply delegates with a null content type.

Tests

Added feign.form.multipart.DelegateWriterTest:

• usesContentTypeFromDelegate — asserts a delegate that sets application/json produces Content-Type: application/json (and not text/plain).
• fallsBackToTextPlainWhenDelegateSetsNoContentType — asserts the text/plain; charset=UTF-8 fallback when the delegate sets no content type.

Test evidence

Ran the form module test suite (and its upstream dependencies) locally:

Tests run: 2, Failures: 0, Errors: 0, Skipped: 0 -- feign.form.multipart.DelegateWriterTest
... full feign-core + feign-form suites green ...
BUILD SUCCESS

Verification done: confirmed the hard-coded text/plain still present on master in SingleParameterWriter#write; confirmed no in-flight PR referencing the issue and no active self-claim (maintainer invited a PR with tests on 2026-05-27); fix touches only .java files; full form module test suite passes after the change.

[velo]

Thanks for the focused fix and for adding coverage around the delegated part content type. I cannot approve this while the required CI check is failing.\n\nGate results:\n1) Test coverage: PASS.\n2) Backwards compatibility: PASS.\n3) Security: PASS.\n\nBlocking issue:\n- Required check is currently failing for this PR. Please get the build green before this can be approved or merged.