refactor(tests): dedupe integration harness, drop dead scaffolding

[RaoulSchaffranek]

Summary

Post-merge cleanup of the test suite. The feature PRs merged today (get-events, tx-result-xdr, simulate-transaction, get-ledger-entries, trace-transaction) each brought their own copy of the integration harness, so the fixtures, RPC plumbing, and deploy helpers had drifted across files. This consolidates them and removes leftover cookiecutter scaffolding.

Changes

• New src/tests/integration/conftest.py — the shared server fixture (was duplicated verbatim in both integration files), the HTTP/JSON-RPC plumbing (_rpc/_post/_post_raw/wat_to_wasm/_find_free_port/_wait_for_server), the spec-shape predicates, and composable transaction helpers (send_tx, fund_account, deploy_contract, make_invoker, deploy_and_get_invoker).
• test_server.py / test_get_events.py import from conftest instead of redefining. The ~7 inline send/builder closures and the five separate deploy helpers collapse onto the shared primitives.
• Unify the duplicated _is_number / _is_json_number predicate.
• Delete komet_node/hello.py (untouched cookiecutter scaffolding, used nowhere in the product) and its two duplicate test_hello tests.
• Strengthen test_call_tx_with_args — it now round-trips each decoded argument from the trace's callContract frame back through scval_from_json and asserts equality with the sent SCVals, so a decoding bug is caught even when the transaction still succeeds. Previously it only asserted SUCCESS.

Net -423/+93 across the edited files, with the shared logic centralised in the new 216-line conftest.

Verification

• make check clean (flake8, mypy, autoflake, isort, black).
• The 78 tests that don't deploy a contract pass — exercising the new conftest fixture, send_tx, fund_account, and the predicate unification.
• No behaviour change: the contract upload/deploy/invoke tests fail only on a pre-existing local toolchain skew (kdist writes compiled.json v3, installed pyk expects v4 → ValueError: Invalid version: 3 in interpreter._inject_program), which reproduces identically on main. CI, running the pinned toolchain, is unaffected.