test: specs for validate-required-config, configuration and IsStellarAddress

[Godbrand0]

Adds the missing unit specs:

• validate-required-config: non-production no-op, each missing secret, insecure JWT default, default/empty DATABASE_URL, aggregated error
• configuration: defaults, empty-string env vars, integer/boolean parsing, NaN for non-numeric PORT/ESCROW_DEADLINE_SECONDS, MAINTENANCE_POOL_CONTRACT_ID fallback
• stellar-address validator: valid G key, bad checksum, M/C addresses, lowercase, non-strings; uses an in-spec StrKey implementation since the jest stellar-sdk mock lacks StrKey

closes #607
Closes #600
Closes #601
Closes #602

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