Fix project scope errors during issue creation - #14035

Fixes #8798

Surface missing ProjectV2 read:project authorization failures when an issue or pull request is explicitly created with --project. Previously, an inaccessible requested ProjectV2 was reported as not found.

• Preserve graceful ProjectV2 degradation for optional metadata, including gh issue view and gh pr view
• Continue resolving matching classic projects even when ProjectV2 discovery lacks read:project
• Keep the existing non-interactive gh project missing-scope output unchanged
• Add regression coverage for issue creation, pull request creation, and deferred project-title resolution

Local demonstration

Fixture-backed checks

From this PR branch, run the focused tests without changing credentials or creating GitHub content:

go test ./pkg/cmd/issue/create -run '^TestIssueCreate_projectMissingReadScope$'
go test ./pkg/cmd/pr/create -run '^Test_createRun/nontty_project_missing_read_scope$'
go test ./api -run '^(TestProjectsV2IgnorableError_missingProjectScope|TestRepoMetadataResultProjectsTitlesToIDsWithMissingProjectsV2Scopes)$'

These exercise an INSUFFICIENT_SCOPES response requiring read:project. They show that explicit issue/PR creation returns actionable scope guidance, while optional ProjectV2 lookup remains ignorable and classic-project resolution still succeeds.

Live comparison with trunk

Use a disposable repository and a token that can access that repository but deliberately lacks read:project. Choose the title of a ProjectV2 that is attached to the repository. Both commands stop during metadata resolution, before creating an issue.

# From this PR checkout
make

# Create a sibling baseline checkout once.
git worktree add ../cli-8798-trunk trunk
(
  cd ../cli-8798-trunk
  make
)

export GH_TOKEN='token-without-read-project'
export DEMO_REPO='OWNER/disposable-repo'
export DEMO_PROJECT='Attached ProjectV2 title'

# Current behavior
../cli-8798-trunk/bin/gh issue create \
  --repo "$DEMO_REPO" \
  --title 'scope demo - baseline' \
  --body 'should not be created' \
  --project "$DEMO_PROJECT"

# Proposed behavior
./bin/gh issue create \
  --repo "$DEMO_REPO" \
  --title 'scope demo - proposed fix' \
  --body 'should not be created' \
  --project "$DEMO_PROJECT"

Captured locally against cli/cli with a token that lacks read:project:

# trunk
could not add to project: 'roadmap' not found

# this PR
error: your authentication token is missing required scopes [read:project]
Update your authentication token to include: read:project

The same comparison applies to gh pr create --project, but the fixture-backed PR test above is preferred because it cannot create a pull request.