fix: strip null optional config fields before validation and include field path in error messages

[devin-ai-integration]

Summary

Fixes production config validation errors ("Config validation error: None is not of type 'string'") affecting 103+ Amazon Seller Partner connections since 2026-06-08. The platform passes explicit null for unset optional config fields; JSON Schema rejects null unless the type includes "null", causing previously-working syncs to fail.

Three changes to check_config_against_spec_or_exit:

1. Strip null optional fields — before validation, remove keys whose value is None and which are not in the schema's required list. Absent keys are not validated by JSON Schema, so this correctly treats null-unset as absent-unset:

def _strip_nulls_from_optional_fields(config, spec_schema):
    required = set(spec_schema.get("required", []))
    return {k: v for k, v in config.items() if v is not None or k in required}

2. Report ALL validation errors — uses Draft7Validator.iter_errors() instead of validate() so all failing fields are reported at once, not just the first:

# Before
Config validation error: None is not of type 'string'

# After (multiple fields)
Config validation error: 'api_token' - None is not of type 'string'; 'region' - None is not of type 'string'

3. Include field path in error messages — uses ValidationError.absolute_path so errors are actionable.

Requested by @gl_anatolii.yatsuk via Metabase error dashboard.

Link to Devin session: https://app.devin.ai/sessions/24208f15bf9f4713bff165f51e1f18b3

[devin-ai-integration]

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[Anatolii Yatsuk (tolik0)]

/prerelease

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[github-actions]

PyTest Results (Fast)

4 100 tests  +6   4 088 ✅ +6   7m 8s ⏱️ -28s
    1 suites ±0      12 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit d0d91d3. ± Comparison against base commit 7da322d.

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PyTest Results (Full)

4 103 tests  +6   4 091 ✅ +6   9m 17s ⏱️ -7s
    1 suites ±0      12 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit d0d91d3. ± Comparison against base commit 7da322d.