Add CRE settings for gateway relay rate limits

[nadahalli]

Summary

• add dedicated CRE settings for confidential relay gateway global and per-node rate limits
• document the new settings in the cresettings README and defaults goldens
• keep the defaults aligned with the current relay handler behavior

Testing

• go test ./pkg/settings/cresettings

[Copilot]

Pull request overview

Adds two new CRE settings to control confidential relay gateway rate limits (global and per-node), with defaults and documentation kept in sync with the existing CRE settings/golden workflow.

Changes:

• Introduce GatewayConfidentialRelayGlobalRate and GatewayConfidentialRelayPerNodeRate in the CRE settings schema and defaults.
• Extend the unmarshal test coverage to validate parsing of the new rate settings.
• Document the new settings in the CRE settings README mermaid diagram and update defaults.{json,toml} goldens.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg/settings/cresettings/settings.go	Adds the two new rate-limit settings to Schema and Default.
pkg/settings/cresettings/settings_test.go	Verifies JSON unmarshal parses the new settings into config.Rate values.
pkg/settings/cresettings/README.md	Documents the new settings in the gateway handler limits diagram.
pkg/settings/cresettings/defaults.toml	Updates TOML golden defaults to include new keys/values.
pkg/settings/cresettings/defaults.json	Updates JSON golden defaults to include new keys/values.

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[github-actions]

✅ API Diff Results - github.com/smartcontractkit/chainlink-common

✅ Compatible Changes (2)

pkg/settings/cresettings.Schema (2)

• GatewayConfidentialRelayGlobalRate — ➕ Added

• GatewayConfidentialRelayPerNodeRate — ➕ Added

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