SHA-256 tests for gix-object

[Sebastian Thiel (Byron)]

As a follow-up to #2497, add SHA-256 tests to gix-object, just because I liked what
happened in gix-diff-tests, and it feels like it's time to have those tests for gix-object
as well.

Related to #2359

Tasks

• refackiew

[Copilot]

Pull request overview

This PR expands gix-object’s test coverage to run against SHA-256 fixtures (via GIX_TEST_FIXTURE_HASH=sha256) in addition to the existing SHA-1 fixture runs, and updates tree-related tests to be hash-kind aware.

Changes:

• Run gix-object unit tests under both SHA-1 and SHA-256 fixture modes from the just workflow.
• Add test helpers to rewrite SHA-1-authored tree fixtures into SHA-256-sized tree data on demand, and normalize tree render output for snapshot reuse.
• Convert tree editor assertions to insta inline snapshots and add a SHA-256 make_trees generated fixture archive.

Reviewed changes

Copilot reviewed 7 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
justfile	Runs gix-object tests twice (SHA-1 and SHA-256 fixture modes), including verbose parsing error mode.
gix-object/tests/object/main.rs	Adds fixture_hash_kind(), tree_fixture(), fixture_oid(), generated_tree_root_id(), and normalize_tree_snapshot() helpers; broadens hash tests to Kind::all().
gix-object/tests/object/tree/iter.rs	Switches to hash-kind aware fixtures and ODB initialization; uses computed root tree id for the fixture hash kind.
gix-object/tests/object/tree/from_bytes.rs	Uses rewritten tree fixtures and hash-kind aware assertions for tree decoding tests.
gix-object/tests/object/tree/editor.rs	Makes editor tests hash-kind aware and replaces string comparisons with insta snapshots (normalized across hash kinds).
gix-object/tests/object/encode.rs	Ensures tree round-trip tests use rewritten tree fixtures under SHA-256 runs; updates test error bounds.
gix-object/tests/fixtures/generated-archives/make_trees_sha256.tar	Adds SHA-256 fixture archive used when GIX_TEST_FIXTURE_HASH=sha256.
gix-object/Cargo.toml	Adds insta dev-dependency and enables gix-hash SHA-256 support for tests.
Cargo.lock	Records the new insta dependency for gix-object tests.