Return dueDate, estimate, and delegate from issues list/search

[FelixLisczyk]

Problem

linear issues list and linear search never return an issue's due date, estimate, or delegate, in either text or JSON output — even when the issue has these values set:

linear issues get TL-563 --output json | jq '.dueDate'
# "2026-08-30"

linear issues list --team TL --output json | jq '.[] | select(.identifier=="TL-563") | .dueDate'
# null

issues get reports all three fields correctly, so this is a query-construction gap in the shared SearchIssuesEnhanced GraphQL query used by both issues list and search, not an API limitation or a downstream rendering bug — the text and JSON renderers already handle these fields correctly once present.

Changes

This PR:

• Adds dueDate, estimate, and delegate { id name email } to the SearchIssuesEnhanced GraphQL query, matching GetIssue's existing selection set.
• Closes the identical gap in ListAssignedIssues and ListAllIssues, which have the same missing-fields pattern.
• Adds DueDate, Estimate, and Delegate fields to core.IssueWithDetails and updates ListAllIssues's response decoding and node-to-result mapping, since it decodes into that type rather than core.Issue directly.
• Updates convertIssueDetails so the three fields survive the IssueWithDetails → core.Issue conversion.
• Adds a shared CapturingTransport test helper (pkg/linear/testutil) that records the outgoing GraphQL request body, and an assertQueryRequestsFields helper, reused across new regression tests for all three fixed query methods.

No changes were needed in internal/format or the JSON DTO layer — both already handled DueDate/Estimate/Delegate correctly once the query returns them.

Testing

• make build and make test pass.
• Manually verified against live Linear data: issues list/search now return dueDate matching issues get for the same issue, in both text and JSON output; issues without a due date continue to omit/null it.
• Adds HTTP-mock regression tests (pkg/linear/issues/client_test.go) for SearchIssuesEnhanced, ListAssignedIssues, and ListAllIssues, asserting both the outgoing query text and the round-tripped field values (including a populated vs. unpopulated case).
• Adds a convertIssueDetails unit test (internal/service/issue_convert_test.go) starting from a fully-populated fixture to confirm the fields survive the conversion.