feat: a fixture value is the shape its format declares

[ShocOne]

Phase 3. Every synthesised string was the attribute path behind a prefix,
whatever the document said the string held:

parsing time "tfpfgen-test-created-at" as "2006-01-02T15:04:05Z07:00"
"tfpfgen-test-agent-id" is not a uuid: invalid UUID format

A generated SDK parses these on the way in — a timestamp becomes time.Time,
an identifier becomes uuid.UUID — so the value was refused before any
assertion in a generated test ran, and the failure named the parse rather than
the field.

ir.Attribute.Format has carried the answer since #91; internal/fixtures
never read it.

What is synthesised

format	value
date-time	2026-01-02T03:04:05Z
date / time	2026-01-02 / 03:04:05Z
uuid	00000000-0000-4000-8000-000000000000
byte / base64	the prefixed name, base64-encoded
email	tfpfgen-test-…@example.invalid
hostname	tfpfgen-test-….example.invalid
uri / url	https://example.invalid/tfpfgen-test-…
ipv4 / ipv6	192.0.2.1 / 2001:db8::1

A format with room keeps the prefix, so a value a test leaves behind on a
live API is still recognisable as toolkit debris and audit cleanup can match
it by name. A timestamp and a uuid have no room; they are fixed instead, which
keeps them deterministic — the whole scheme rests on a regenerated fixture being
byte-identical.

The reserved ranges are deliberate: TEST-NET-1 and example.invalid exist for
exactly this, so a value that escapes into a request reaches nothing real.

Measured effect

Every generated list-resource test now passes, on all three pilots:

pilot	before #99	after #99	after #100	now
github	23	23	20	0
jamfpro	39	39	5	0
thousandeyes	11	11	3	0

unsupported.json unchanged on all three (835 / 255 / 361) — this changes
fixture values, not what generates. provider verify reports no drift
(4384 / 3895 / 1798 files).

What this uncovers

With the parse failures gone, the datasource suites show their own next family —
167 occurrences of:

Error: Value Conversion Error
mismatch between struct and object: Struct defines fields not found in object: org

Masked until now behind the schema-load failure #99 fixed. That is Phase 4 and
is not attempted here.

Gates

• gofmt, repo_hygiene_gate.sh, golangci-lint (0 issues), go build/go vet — pass
• go test ./... — pass
• provider generate + provider verify on all three — no drift

New test TestUnit_Fixturespec_AFormatDecidesTheValueShape checks each shape
parses (time.Parse, netip.ParseAddr, a uuid shape match — the toolkit takes
no uuid dependency), that the prefix survives where it can, and that a second
derivation is identical.

🤖 Generated with Claude Code