fix(epp): add opt-in gateway protocol compatibility

[Oxygen56]

Overview:

Add an opt-in compatibility mode for gateways such as agentgateway that stream request bodies without waiting but omit the corresponding ext_proc capability flag. Strict validation remains the default so gateways that really wait for the header response still fail fast instead of deadlocking.

Details:

• Add DYN_EPP_ALLOW_MISSING_SEND_BODY_WITHOUT_WAITING, defaulting to false.
• When enabled, waive only the missing capability flag; request and response body modes must still both be FULL_DUPLEX_STREAMED.
• Enable the compatibility mode in the repository's agentgateway on-ramp manifest, where the gateway's request-body behavior is known.
• Emit a startup warning and document the safety boundary for operators.
• Add focused policy and gRPC stream regression coverage for strict and compatibility behavior.

Where should the reviewer start?

Start with the protocol validation and regression coverage in deploy/inference-gateway/ext-proc/src/server.rs, then review the opt-in wiring in deploy/inference-gateway/ext-proc/src/runner.rs and the affected deployment setting in deploy/inference-gateway/ext-proc/examples/onramp/agg.yaml.

Validation

• A source-bound before/after regression using the affected on-ramp deployment proved that the reported configuration is rejected before the deployment setting and accepted after it, while unsafe body modes remain rejected.
• The complete dynamo-ext-proc test suite passed, including the gRPC compatibility stream regression.
• Rust formatting and strict lint checks passed; the updated multi-document Kubernetes manifest parsed successfully and the diff has no whitespace errors.
• No live agentgateway/vLLM cluster was available; the focused checks exercise the exact deployed setting and EPP stream boundary without one.

Related Issues

🔗 This PR is linked to an issue:

• Closes [BUG]: Standalone Rust EPP fails with agentgateway because ProtocolConfiguration misses send_body_without_waiting #13686

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👋 Hi Oxygen56! Thank you for contributing to ai-dynamo/dynamo.

Just a reminder: The NVIDIA Test Github Validation CI runs an essential subset of the testing framework to quickly catch errors.Your PR reviewers may elect to test the changes comprehensively before approving your changes.

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📥 Commits

Reviewing files that changed from the base of the PR and between 004cd02 and d10e3cf.

📒 Files selected for processing (3)

• deploy/inference-gateway/ext-proc/DEVEL.md
• deploy/inference-gateway/ext-proc/src/runner.rs
• deploy/inference-gateway/ext-proc/src/server.rs

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Walkthrough

Changes

Protocol compatibility override

Layer / File(s)	Summary
Server option and protocol validation deploy/inference-gateway/ext-proc/src/server.rs	ExtProcServer adds an opt-in compatibility setting. Validation accepts a missing streaming flag only when both body modes are full-duplex.
Runtime configuration wiring deploy/inference-gateway/ext-proc/src/runner.rs, deploy/inference-gateway/ext-proc/DEVEL.md	The runner reads DYN_EPP_ALLOW_MISSING_SEND_BODY_WITHOUT_WAITING, logs when enabled, and configures the server. Documentation records the setting and its disabled default.
Compatibility validation coverage deploy/inference-gateway/ext-proc/src/server.rs	Tests cover strict rejection, invalid body modes, compatibility-mode streams, and successful request routing.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to d10e3

This adds an opt-in gateway compatibility mode while keeping strict validation as the default, limiting the behavior change to explicitly enabled deployments. No actionable merge-blocking risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes address issue #13686 by adding opt-in compatibility for full-duplex gateways while preserving strict validation by default.
Out of Scope Changes check	✅ Passed	The documentation, runtime wiring, validation changes, and regression tests all support the linked issue and stated objectives.
Docstring Coverage	✅ Passed	Docstring check was indeterminate for this PR — some files could not be analyzed in time. Not blocking.
Description check	✅ Passed	The description includes all required template sections, explains the compatibility mode, identifies review starting points, and links issue #13686.
Title check	✅ Passed	The title clearly and concisely describes the opt-in gateway protocol compatibility change.

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