fix(bundler): treat an explicit-null manifest field as missing, not the text "None"

[jawwad-ali]

Problem

BundleManifest.from_dict reads every required scalar as str(raw.get(key, "")).strip(). The "" default only covers a missing key. A key that is present but null — exactly how YAML spells an empty field — yields None, and str(None) is the literal string "None". That value is non-empty, so it sails straight past the if not value required-field checks in structural_errors().

bundle:
  id: demo
  name: Demo
  version: 1.0.0
  role: developer
  description:      # <- empty
  author:           # <- empty
  license:          # <- empty

Reproduced on main:

description='None'   author='None'   license='None'
structural_errors() == []
specify bundle validate -> exit 0: "demo is well-formed and valid."

So an empty required field is silently accepted, and the bundle ships the literal text "None" as its author/license/description — which is what bundle info and a catalog entry then display to users. A null provides.<kind>[].id likewise became a component literally named "None".

Fix

Add a _text() helper beside the existing _parse_str_list — matching this file's established "one coercion helper, applied at every site" shape — that maps an explicit null to "", and route the required scalars (schema_version, the six bundle.* fields, requires.speckit_version, component id) through it.

After:

description=''  author=''
structural_errors():
  - Missing required field: bundle.description.
  - Missing required field: bundle.author.
  - Missing required field: bundle.license.

This is the same silent-acceptance class as the already-merged guards in this exact function — #3629 (present-but-non-mapping integration: was silently dropped) and #3661 (falsy non-mapping requires/provides).

Not a behaviour change for valid input

_text is equivalent to the code it replaces for every input except an explicit None: an absent key already produced "", and non-null scalars are still str()-coerced and stripped. Verified — a valid manifest still yields structural_errors() == [] with author='Ali'.

Verification

• test_explicit_null_bundle_field_is_reported_as_missing (5 parametrized fields) + null speckit_version + null component id: 7 fail before, pass after.
• tests/contract/test_manifest_schema.py: 35 passed (28 pre-existing untouched).
• tests/contract/ + tests/unit/: 272 passed — the 3 remaining failures are pre-existing Windows symlink-privilege tests (need admin; pass on CI's Linux/macOS runners).
• uvx ruff@0.15.0 check src tests clean.

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AI-assisted: authored with Claude Code. I reproduced the "None" values and the clean bundle validate on main first, then confirmed valid manifests are unaffected after the change.