feat(client): add getBillCodes (1.5.0)

[eyuapptree]

Summary

• Add `FamisClient.getBillCodes(context)` for `GET /billcodes`
• Add `BillCode` model
• Bump version to 1.4.2 and update CHANGELOG

Test plan

• `tsc --noEmit` passes
• Verify against a live tenant

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[alexisandreason]

Code review

Fix before merging

• Duplicate BillCode interface (model/famis_models.ts ~L1659). An identical BillCode interface already exists further down the same file (L2459 on master). The two declarations merge silently today, but tsc will fail with "Subsequent property declarations must have the same type" the first time one copy diverges. Delete the new block and use the existing interface.
• Version bump should be 1.5.0, not 1.4.2 (package.json, CHANGELOG.md). getBillCodes is new public API logged under "Added". The CHANGELOG's stated SemVer policy and repo precedent (1.4.0 "Added" was a minor bump, 1.4.1 "Fixed" a patch) call for a minor bump. Retitle the CHANGELOG section [1.5.0].

Cleanup suggestions

• Placement (famis_client.ts ~L645). getBillCodes sits between getPayPeriods and the //region TimeZones block, ~440 lines away from getPropertyBillCodeAssociations (~L1080). Move it next to that getter or give it its own //region BillCodes.
• No test. tests/integration.test.ts has a "Lists" describe block with one should fetch all <x> case per simple getter; getBillCodes is not added. The 'billcodes' route and BillCode field names are unverified against a live tenant (note Active here vs ActiveFlag on the sibling PropertyBillCodeAssociations model), and the PR's "Verify against a live tenant" box is unchecked.
• UpdateDate: Date (model/famis_models.ts ~L1662). getAll returns raw JSON, so this is an ISO string at runtime and billCode.UpdateDate.getTime() throws despite a clean tsc. This matches a pre-existing pattern in many models, but the PR adds another instance; string would be honest.

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[eyuapptree]

Thanks for the review, all addressed in ce819fb and e464a48:

• Duplicate BillCode: removed the new copy and kept the existing interface.
• Version: bumped to 1.5.0 in package.json and the CHANGELOG heading, and retitled the PR.
• Placement: moved getBillCodes into a new //#region bill codes, next to getPropertyBillCodeAssociations.
• Test: added should fetch all bill codes to the "Lists" block. I haven't run it against a live tenant yet, so the verify box is still unchecked.
• UpdateDate: changed to string on the existing BillCode model.

tsc is clean.

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