fix: add maximum size validation for JWT secrets to prevent DoS attacks

[crStiv]

Add maximum size validation for JWT secrets in eth1_api auth module

• Add JWT_SECRET_SIZE_MAX_BYTES constant (1024 bytes) to prevent DoS attacks
• Update size validation to check both minimum (32 bytes) and maximum (1024 bytes)
• Improve error message to indicate valid size range
• Add test case for large JWT secret rejection

[CLAassistant]

All committers have signed the CLA.

[Tumas]

Hey,

Thanks for the pull request! A few notes:

• The secret is configured and read from the local filesystem, not exchanged over the network, so a DoS attack does not apply in this context.
• According to the current spec, the secret key is expected to be 256 bits. If it is not, clients should return an error. Previously, the requirement was “at least 256 bits,” which explains the current implementation.

Feel free to adjust your pull request or open a new one to enforce an exact 256-bit check for the secret if you prefer.

[crStiv]

@Tumas made some changes, hope I implemented everything exactly as you wished it to be done

[crStiv]

@Tumas pinging