fix(sdk): reject Content-Type in default headers

[devin-ai-integration]

Summary

Reject Content-Type in S2Config::with_default_headers, alongside the existing encoding/framing restrictions. The SDK chooses the protocol for each operation: a default Content-Type: s2s/proto can make a unary read receive streaming frames, causing a decode error or timeout.

This addresses #737 at configuration validation, as an alternative to the per-read override in #738. All Content-Type values are rejected because request format belongs to the SDK.

Extend the existing rejection tests to cover S2S, protobuf, JSON, mixed-case header names, and empty values. Remove the now-invalid Content-Type default from the header propagation fixture while preserving its assertions.

Validation: the five new cases failed before the fix. just test passes all 796 workspace tests; SDK clippy with all features/targets and just fmt also pass.

Closes #737

Link to Devin session: https://app.devin.ai/sessions/c3dea6e292ce4071a41baf4943f1557c
Open in Devin Desktop: https://app.devin.ai/desktop/session/c3dea6e292ce4071a41baf4943f1557c?variant=devin
Requested by: @sgbalogh

[devin-ai-integration]

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[greptile-apps]

Greptile Summary

This PR prevents hidden SDK configuration from supplying a default Content-Type, ensuring each operation retains control of its request protocol.

• Adds Content-Type to the headers rejected by S2Config::with_default_headers.
• Tests rejection across S2S, protobuf, JSON, mixed-case, and empty values.
• Updates the default-header propagation fixture to comply with the tightened validation.

Confidence Score: 5/5

The PR appears safe to merge with no actionable correctness, security, or repository-rule issues identified.

The validation consistently rejects case-insensitive Content-Type defaults before request construction, tests the relevant value variants, and leaves operation-specific Content-Type selection intact.

Important Files Changed

Filename	Overview
sdk/src/types.rs	Extends default-header validation and its parameterized tests to reject every Content-Type value.
sdk/src/api.rs	Removes the newly invalid Content-Type entry from the header-propagation fixture while retaining verification of the SDK-selected streaming type.

_{Reviews (1): Last reviewed commit: "fix(sdk): reject Content-Type in default..." | Re-trigger Greptile}