parser improvements: use utils to access discriminators; harden accounts access

[grooviegermanikus]

Summary

Use hardened parse utils to work with discriminators.
Refactor accounts access from index style to let-match.

Audit Status

• [*] Required — Audit completed and artifacts attached
• Not Required — Justification below (3–5 lines)

If Not Required (Justification)

Trivial and mechanical refactoring.,

[github-actions]

Coverage report

Filename	Lines Covered (%)	Functions Covered (%)
order-engine-sdk/src/fill.rs	95.98	86.11
order-engine-sdk/src/lib.rs	66.67	50
order-engine-sdk/src/parse_util.rs	99	100
order-engine-sdk/src/transaction.rs	0	0
programs/order-engine/src/error.rs	100	100
programs/order-engine/src/instructions/fill.rs	98.29	80
programs/order-engine/src/lib.rs	90.91	75
server-example/src/config.rs	0	0
server-example/src/server.rs	0	0
squads-sdk/src/accounts.rs	93.51	100
squads-sdk/src/config.rs	39.68	50
squads-sdk/src/error.rs	0	0
squads-sdk/src/lib.rs	100	100
squads-sdk/src/pda.rs	98.04	100
squads-sdk/src/serialize.rs	70.83	60
squads-sdk/src/settings.rs	94.02	100
squads-sdk/src/transaction.rs	44.33	44.44
squads-sdk/src/unwrap.rs	89.43	71.43
squads-sdk/src/wrap.rs	84.28	85.71
webhook-api/src/enums.rs	88.75	18.18
webhook-api/src/requests.rs	0	0
webhook-api/src/responses.rs	0	0