build(llm-router): allow always-disagg deciders in llm-d-router observer

[thxCode]

What type of PR is this?

/kind enhancement
/area worker

What this PR does / why we need it:

• Adds always-disagg-pd-decider and always-disagg-multimodal-decider to the observer plugin allowlist (SupportedPlugins) and unit test coverage (TestObserverPluginCoverage) in pack/llm-router/patches/llm-d/0001-observer-endpoints-and-dispatch.patch.
• Without these two built-in llm-d-router deciders in the observer's known plugin set, /observer/endpoints refuses requests with 403 Forbidden when llm-d-router runs an Encoder-Prefill-Decode (always-disagg-multimodal-decider) or SGLang disaggregated (always-disagg-pd-decider) profile.

Which issue(s) this PR links to:

NONE

Special notes for your reviewer:

• Follow-up to build(llm-router): bump llm-d-router to v0.11.0 and patch vllm-router for EPD #761 so the Base Image workflow can publish gpustack/llm-router:v0.3.1 before the operator's default router image setting is bumped.
• Verified that 0001-observer-endpoints-and-dispatch.patch applies cleanly via git apply --check against llm-d/llm-d-router@v0.11.0.

Does this PR introduce a user-facing change?

NONE

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