test(client): bar-raising coverage for GovParamChangeTask (PLT-487)

[bdchatham]

Follow-up to #203. The gov-param-change handler was tested, but the client GovParamChangeTask builder had no coverage (flagged in cross-review). Adds:

• Validate() table — valid + every failure branch.
• ToTaskRequest() — type + required param keys + memo-omitted-when-empty.
• ValueSingleEncodedOnWire — the client-side half of the single-encode contract: the per-change value survives ToTaskRequest + the HTTP json.Marshal byte-identical for object/string/number/bool, complementing the handler-side regression guard. This is the full client→wire path the prior unit test bypassed.

Test-only; no behavior change.

🤖 Generated with Claude Code

[cursor]

PR Summary

Low Risk
New unit tests only; no production code or runtime behavior changes.

Overview
Adds sidecar/client/gov_param_change_test.go to cover the client GovParamChangeTask builder, which previously had no tests while the handler was already covered.

Validate() — asserts a valid fixture passes and table-driven cases fail for each required field (chain, key, title, description, changes, deposits, fees, gas).

ToTaskRequest() — checks task type, required param keys, and that empty memo is omitted like sibling tasks.

ValueSingleEncodedOnWire — marshals params as the HTTP client would and asserts each change value stays byte-identical for object/string/number/bool JSON, guarding the client-side single-encode contract (complement to handler tests for the prop-252 double-encode issue).

Test-only; no runtime behavior change.

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