[alex-leb]

Adds the missing deprecation warning to Domains#get_domain_stats, and moves every deprecation warning in the gem to one format that names the replacement and points at the caller's code.

Missing warning

Mailgun's API docs deprecate three APIs the gem calls. get_domain_stats was the only wrapper that didn't warn.

Gem method	Endpoint	Deprecated in favor of	Warned before
Domains#get_domain_stats	GET /v3/{domain}/stats/total	Metrics API (Metrics#account_metrics)	No
Tags#get_tags and the other Tags methods	GET /v3/{domain}/tags	Tags API (AnalyticsTags)	Yes
Events#get, #next, #previous	GET /v3/{domain}/events	Logs API (Logs#account_logs)	Yes

One warning format

The new Mailgun::Deprecation.warn prints:

app/jobs/report.rb:12: warning: DEPRECATION: "Mailgun::Tags#get_tag" is deprecated (Mailgun deprecated the Tags API) and will be removed in mailgun-ruby 1.5. Please use "Mailgun::AnalyticsTags#list" instead.

• The location is the first caller outside the gem. It skips the gem's own frames, including the ApiVersionChecker wrappers, so it points at the line to change.
• The reason only appears where Mailgun deprecated the API, and the removal clause only where a removal is planned.
• It's used by Domains#get_domain_stats, Events, all 8 Tags methods, the 7 Webhooks aliases and the 7 MessageBuilder setters. The two plain warn calls left (the yes/no value hint and the API version check) aren't deprecations.

Message changes

• Tags: each method now names its replacement (AnalyticsTags#list, #update, #remove or Metrics#account_metrics) and says it will be removed in 1.5, as the existing TODO: remove in v 1.5 plans. Before, all 8 printed the same line with no replacement.
• Webhooks aliases: "will be deprecated in future versions of Mailgun" is now "is deprecated and will be removed in a future version of mailgun-ruby". The aliases are already deprecated, and only the gem's old method names are going away, not the endpoints.
• MessageBuilder setters: same wording as the Webhooks aliases.
• The warning text changes, so anything matching the old strings needs updating.

Docs

• docs/Snippets.md used the deprecated set_from_address, set_subject, set_text_body and set_message_id; it now uses from, subject, body_text and message_id.
• docs/Events.md names Logs#account_logs as the replacement.

Testing

• New specs for the helper (message format, and the caller location through gem frames and the API version wrappers) and for all 8 Tags warnings, which had no specs.
• The MessageBuilder deprecation specs check the printed warning instead of stubbing warn; the Webhooks and Domains specs match the new wording.
• Full suite, run without credentials: 438 examples, 0 failures; line coverage 96.58%; RuboCop clean.