Add user message wire contract

[atavism]

For https://github.com/getlantern/engineering/issues/3784

[coderabbitai]

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[Copilot]

Pull request overview

Adds a new usermessage package that defines and validates a v1 JSON wire contract for fetching resolved in-app user messages, along with embedded fixtures and compatibility/validation tests.

Changes:

• Introduces v1 request/response/message/action types with explicit wire-size limits.
• Adds validation for request, response, resolved message content, and allowlisted actions (including HTTPS URL validation).
• Adds versioned embedded JSON fixtures and tests covering round-trip JSON compatibility and validation rules.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
usermessage/types.go	Defines the v1 wire-contract structs, enums, and size-limit constants.
usermessage/validation.go	Implements validation for the v1 request/response/message/action contract.
usermessage/types_test.go	Adds fixture round-trip tests and targeted validation/limits tests.
usermessage/testfixtures/fixtures.go	Exposes embedded, versioned JSON fixtures via an embed.FS.
usermessage/testfixtures/v1/request.json	Adds a v1 request fixture used for compatibility tests.
usermessage/testfixtures/v1/response_empty.json	Adds a v1 “no message” response fixture.
usermessage/testfixtures/v1/response_message.json	Adds a v1 response fixture containing a message with an HTTPS URL action.
usermessage/testfixtures/v1/response_no_action.json	Adds a v1 response fixture containing a message without an action.
usermessage/testfixtures/v1/response_open_plans.json	Adds a v1 response fixture containing a message with an open_plans action.

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