fix(validation): validate managementToken format using regex

[Abfa41]

Description

Fixes #6192

This PR adds format validation for the managementToken field to ensure malformed tokens are rejected during schema validation instead of failing later during authentication.

Changes made

• Added regex validation for managementToken.
• Enforced the required cpn_ prefix.
• Restricted token characters to base64url-safe characters (A-Z, a-z, 0-9, _, -).
• Preserved existing length constraints and optional behavior.
• Added a clear validation error message for malformed tokens.
• Updated notification route test fixtures to use management tokens that match the enforced cpn_ token format.

Benefits

• Provides immediate feedback for invalid tokens.
• Improves debuggability for developers.
• Reduces confusion caused by generic authentication failures.
• Prevents malformed values from progressing further into the application.

Validation Rules

Accepted format:

cpn_xxxxxxxxxxxxxxxxx

Rejected examples:

abc_token
cpn token
cpn@token

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📦 Next.js Bundle Size Report (Gzipped Sizes)

Asset	PR Size	Base Size	Difference	Status
static/chunks/9389-[hash].js	6.25 KB	6.19 KB	+0.07 KB (+1.12%)	🔴 Regression

📊 Summary of Totals

Category	PR Size	Base Size	Difference
Total JS	3710.74 KB	3710.63 KB	+0.11 KB (+0.00%)
Total CSS	290.64 KB	290.64 KB	0 B