[Story] Implement Trial balance report

[markheydon]

Summary

Implement the FreeAgent Trial balance report endpoint in the SDK.

Why

Layer 7 date-bounded trial balance. See reports entity map.

Scope

In scope:

• Typed models, wrappers, and service method(s) per https://dev.freeagent.com/docs/trial_balance
• Tests, sample app probe, API coverage

Out of scope:

• Other report pages (siblings)

Acceptance criteria

• Implemented per FreeAgent docs
• Follows implement-endpoint and sample probe standards
• Reports entity map SDK column updated

Parent

• [Feature] Implement Reports cluster resources #68 — Implement Reports cluster resources

Goal links

• G1 (strongly typed SDK)
• G2 (developer experience)

Sequencing

After operational clusters.