sonic: Fix STATIC_ROUTE clobber in OOB management path

[ideaship]

Summary

• generate_sonic_config() reset STATIC_ROUTE to {} before writing the management default route, silently discarding any routes loaded from /etc/sonic/config_db.json on every sync.
• The reset was introduced in f4f2290 when base config loading did not exist yet — it was harmless at the time and went unnoticed when base config loading was added later.
• Fix: write the management route directly into the existing dict without resetting it first.

Test plan

• Verify existing unit tests pass (pytest tests/unit/tasks/conductor/sonic/)
• Add a test that pre-populates STATIC_ROUTE in the base config and asserts entries survive the OOB management path (tracked as follow-up in PR Add unit tests for SONiC config_generator orchestrator and service ca… #2237 review)

🤖 Generated with Claude Code

[sourcery-ai]

Hey - I've left some high level feedback:

• When writing to config["STATIC_ROUTE"]["mgmt|0.0.0.0/0"], consider using config.setdefault("STATIC_ROUTE", {})[...] or otherwise ensuring the key is initialized so the function remains robust if called with a config that lacks a STATIC_ROUTE section.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- When writing to `config["STATIC_ROUTE"]["mgmt|0.0.0.0/0"]`, consider using `config.setdefault("STATIC_ROUTE", {})[...]` or otherwise ensuring the key is initialized so the function remains robust if called with a config that lacks a `STATIC_ROUTE` section.

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