handle buffer space in rc_client image URL functions

[stenzek]

Since there is a status code return, indicate that the buffer was too small rather than silently truncating the URL.

[Copilot]

Pull request overview

This PR improves buffer handling in rc_client image URL functions by returning RC_INSUFFICIENT_BUFFER when the provided buffer is too small, instead of silently truncating URLs using snprintf. The implementation replaces snprintf with explicit length checking and memcpy for exact string copying.

Changes:

• Modified five image URL functions to check buffer size before copying and return RC_INSUFFICIENT_BUFFER if insufficient
• Added test coverage for buffer size validation in three test functions
• Ensures null terminator space is properly accounted for in buffer size checks

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/rc_client.c	Updated five functions (rc_client_get_image_url, rc_client_user_get_image_url, rc_client_game_get_image_url, and two paths in rc_client_achievement_get_image_url) to validate buffer size and return RC_INSUFFICIENT_BUFFER instead of silently truncating
test/test_rc_client.c	Added buffer size validation tests for user, game, and achievement image URL functions

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