Convert `from` to `try_from` for `Certificate` and `CertificateRecord`

[dlachaume]

Why

The conversion between Certificate and CertificateRecord is fallible: conversions operations can fail (e.g. to_json_hex()).

Currently, these conversions are implemented using From with unwrap(). Switching to TryFrom makes failures explicit and allows for proper error propagation.

What

Update the conversions in mithril-aggregator/src/database/record/certificate.rs:

• Change impl From<Certificate> for CertificateRecord to TryFrom
• Change impl From<CertificateRecord> for Certificate to TryFrom

How

• Replace From with TryFrom in both directions between Certificate and CertificateRecord
• Update all usages to use try_into() and handle failures