Unify payload size threshold validation across TOML and stdin config paths

[Copilot]

gateway.payload_size_threshold (TOML) and gateway.payloadSizeThreshold (stdin JSON) were validated in two places with different bounds, so 0 was accepted in TOML but rejected in stdin. This change consolidates the rule and makes both config surfaces reject explicit non-positive values consistently.

• What changed

  • Added a shared rules.PositiveInteger validator for config fields that must be >= 1.
  • Replaced the ad-hoc payloadSizeThreshold / payload_size_threshold checks in both validation paths with the shared rule.
  • Preserved existing defaulting behavior for omitted values while rejecting explicitly configured 0 on the TOML path.

• TOML path

  • Validates gateway.payload_size_threshold before defaults are applied.
  • Uses TOML metadata to distinguish omitted from explicitly set to 0, so:
    • omitted field still defaults
    • explicit 0 now fails validation instead of being silently normalized

• stdin / JSON path

  • Uses the same shared validator for gateway.payloadSizeThreshold.
  • Keeps error structure and semantics aligned with the TOML path.

• Tests

  • Extended TOML coverage to reject both 0 and negative values.
  • Added direct unit coverage for the shared positive-integer rule.

if md.IsDefined("gateway", "payload_size_threshold") {
	if err := rules.PositiveInteger(
		cfg.Gateway.PayloadSizeThreshold,
		"payload_size_threshold",
		"gateway.payload_size_threshold",
	); err != nil {
		return nil, err
	}
}

if gateway.PayloadSizeThreshold != nil {
	if err := rules.PositiveInteger(
		*gateway.PayloadSizeThreshold,
		"payloadSizeThreshold",
		"gateway.payloadSizeThreshold",
	); err != nil {
		return err
	}
}

[Copilot]

Pull request overview

This PR consolidates validation for the gateway.payload_size_threshold (TOML) and gateway.payloadSizeThreshold (stdin JSON) configuration fields so both surfaces consistently reject explicitly configured non-positive values while preserving defaulting behavior when the field is omitted.

Changes:

• Introduces a shared rules.PositiveInteger validator and uses it for stdin gateway config validation.
• Moves TOML payload_size_threshold validation to occur before defaults are applied, using TOML metadata to distinguish omitted vs explicitly-set values.
• Extends/updates tests to cover invalid payload_size_threshold values and the new shared rule.

Show a summary per file

File	Description
internal/config/validation.go	Switches stdin payloadSizeThreshold validation to the shared positive-integer rule.
internal/config/rules/rules.go	Adds PositiveInteger validator used across config validation paths.
internal/config/rules/rules_test.go	Adds unit tests for the new PositiveInteger validator.
internal/config/config_core.go	Validates TOML payload_size_threshold before defaults (only when explicitly defined).
internal/config/config_core_test.go	Updates TOML tests to reject both 0 and negative payload_size_threshold values.

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• Files reviewed: 5/5 changed files
• Comments generated: 1