deps: Update schemars to 1.x

[kejadlen]

Upgrades schemars from 0.8 to 1.2 and adapts to API changes.

API changes:

• schemars::gen module renamed to schemars::generate
• option_nullable setting removed—schemars 1.x generates type: ["string", "null"] for Option types natively

Schema compatibility fixes:

• Added skip_serializing_if to #[schemars(skip)] fields that were serializing to null but falling under additionalProperties: { type: "string" }
• Removed deny_unknown_fields from ReportDestinationTemplates which conflicted with its flattened BTreeMap

Test fix:

• Added mutex to serialize environment variable tests in directories.rs to prevent flaky failures from test parallelism

Assisted-by: Claude Opus 4.5 via Claude Code