hclwrite: body.SetAttribute* return new attribute

[bbasata]

Description

This change updates hclwrite.Body.SetAttribute* to return the new attribute rather than return nil.

This matches the intent in the documentation comments that indicate:

The return value is the attribute that was either modified in-place or created.

An available workaround:

body.SetAttributeRaw(name, tokens)
attr := body.GetAttribute(name)

Open question: I'm considering whether to check the return value of Body.SetAttribute* in hclwrite/ast_body_test.go.

[bbasata]

TIL: #613

[bbasata]

Tests have been ... driven! These fail on main, as they should.

$ git checkout -b one main
Switched to a new branch 'one'
$ git cherry-pick bbasata/set
[one d135622] hclwrite: unit testing for Body.SetAttributeRaw
 Date: Wed Jul 29 13:11:25 2026 -0400
 1 file changed, 122 insertions(+)
$ go test ./hclwrite
--- FAIL: TestBodySetAttributeTraversal_ReturnsTheAttribute (0.00s)
    --- FAIL: TestBodySetAttributeTraversal_ReturnsTheAttribute/attribute_`one`_is_not_set_to_a_value (0.00s)
        ast_body_test.go:800: got: nil
            want: one=the.loneliest.number
--- FAIL: TestBodySetAttributeRaw_ReturnsTheAttribute (0.00s)
    --- FAIL: TestBodySetAttributeRaw_ReturnsTheAttribute/attribute_`one`_is_not_set_to_a_value (0.00s)
        ast_body_test.go:996: got: nil
            want: one="the loneliest number"
--- FAIL: TestBodySetAttributeValue_ReturnsTheAttribute (0.00s)
    --- FAIL: TestBodySetAttributeValue_ReturnsTheAttribute/attribute_`one`_is_not_set_to_a_value (0.00s)
        ast_body_test.go:1027: got: nil
            want: one="the loneliest number"
FAIL
FAIL    github.com/hashicorp/hcl/v2/hclwrite    0.570s
FAIL

[mildwonkey]

This is the nittiest of nitpicks I can pick (and I'm genuinely sorry for it), but what do you think about naming this something different? testFn instead of must, maybe?

We use "must" in terraform test fixtures to mean "this will panic if it fails" and "test" for test helpers, and while this obviously isn't terraform, consistency simplifies reviewing.

[bbasata]

test + must -> trust? :)

[bbasata]

Yes, I typically want a must function to panic. In this case, I extracted an existing "do something, and t.Fatal on diagnostics" pattern from this file.