fix: unescape literal \n sequences in Telegram message formatting

[ptone]

Summary

• Adds unescapeNewlines helper that replaces literal \n and \t escape sequences with actual newline and tab characters
• Applies the helper in both FormatMessage and FormatMessageV2 before writing msg.Msg to the output buffer
• Message text can arrive with literal escape sequences when it passes through JSON encoding (e.g. FormatForDelivery) and is forwarded without decoding, or when shell arguments carry un-interpreted backslash escapes

Closes #48

Test plan

• Added TestUnescapeNewlines with 7 cases covering no escapes, single/double newlines, tabs, mixed, actual newlines unchanged, and empty string
• Added TestFormatMessage_UnescapesLiteralNewlines verifying FormatMessage output contains actual newlines
• Added TestFormatMessageV2_UnescapesLiteralNewlines verifying FormatMessageV2 output contains actual newlines
• Full telegram test suite passes (all existing tests unaffected)

[gemini-code-assist]

Code Review

This pull request introduces a helper function unescapeNewlines to replace literal escape sequences (\n and \t) with their actual characters in formatted Telegram messages, and adds corresponding unit tests. Feedback suggests optimizing this helper by declaring the strings.Replacer as a package-level variable to avoid the overhead of recreating it on every function call.

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[gemini-code-assist]

Creating a new strings.Replacer on every call to unescapeNewlines is inefficient because strings.NewReplacer performs preprocessing to build its internal lookup structures. Declaring the replacer as a package-level variable avoids this overhead and improves performance.

Suggested change

	// unescapeNewlines replaces literal escape sequences (\n, \t) with their actual
	// characters. Message text may arrive with these sequences when it passes through
	// JSON encoding (e.g. FormatForDelivery) and is later forwarded without decoding,
	// or when shell arguments carry un-interpreted backslash escapes.
	func unescapeNewlines(s string) string {
	return strings.NewReplacer(`\n`, "\n", `\t`, "\t").Replace(s)
	}
	var unescapeReplacer = strings.NewReplacer("\\n", "\n", "\\t", "\t")
	// unescapeNewlines replaces literal escape sequences (\n, \t) with their actual
	// characters. Message text may arrive with these sequences when it passes through
	// JSON encoding (e.g. FormatForDelivery) and is later forwarded without decoding,
	// or when shell arguments carry un-interpreted backslash escapes.
	func unescapeNewlines(s string) string {
	return unescapeReplacer.Replace(s)
	}