packetbeat/otel: restore per-stream interface and procs from hybrid agent config

[efd6]

Proposed commit message

packetbeat/otel: restore per-stream interface and procs from hybrid agent config

When elastic-agent runs packetbeat as an OTel receiver it delivers config
in a hybrid format: the top-level key is protocols (beat-native) but each
list entry contains agent-style per-stream fields including interface,
procs, and data_stream. FromStatic read Interfaces only from the top level
and Procs not at all, so every per-stream interface device and monitored
process list was silently dropped and packetbeat fell back to defaultDevice()
on every deployment.

Extend the protocol-routing loop in FromStatic to also unpack interface and
procs from each entry, matching the approach already used by NewAgentConfig
for the process runtime. Add a defaultDevice() fallback for the case where
neither top-level nor per-stream interfaces are present.

Add a parity test with 16 pre-rendered stream fixtures for the
network_traffic integration.

Note

This is the first of four changes to add behaviour parity checks between the two configuration pathways.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

Disruptive User Impact

How to test this PR locally

Related issues

• Fixes packetbeat: per-stream interface and procs configuration silently dropped under OTel runtime #53010
• Closes packetbeat: use management.UnderAgent() for OTel receiver config selection #53050

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