chore: Prepare Release 4.15.0

[superchilled]

This PR prepares a new release of the SDK by incrementing the minor version from 14 to 15 and updating the changelog.

[vonage-ai-assistant]

AI Adoption Report

	Lines	%
AI-generated	0	0%
Human	0	100%
Untracked	6	—
Total	6

Per-commit breakdown (2 commits)

Commit	Message	Added	AI	Human	AI %
6cfb12c	Updating changelog	5	—	—	untracked
5046863	Bumping minor version	1	—	—	untracked

Warning

The following commits have no Git AI authorship notes — AI adoption cannot be tracked for these changes.
Install git-ai locally (curl -sSL https://usegitai.com/install.sh | bash) to enable automatic tracking.
An automation that cannot run git-ai can declare its own attribution instead, by putting #ai-attribution-NNN in the commit message, where NNN is a whole number from 0 to 100.

• 6cfb12c Updating changelog
• 5046863 Bumping minor version

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_{Powered by Git AI Standard v3.0.0 — authorship data from refs/notes/ai (supports sessions-v2 format)}