Preserve Jira ADF issue descriptions

[eshulman2]

Summary

• expand JiraIssue ADF text extraction to preserve headings, lists, code blocks, hard breaks, and rules
• keep the extractor local with a guardrail comment instead of adding an under-supported ADF converter dependency
• preserve raw ADF node representation when an unsupported node cannot be parsed, so workflow prompts do not silently lose issue content
• add regression tests covering Epic plan content round-tripped through Markdown -> ADF -> JiraIssue.description and unknown ADF fallback behavior

Verification

• proved the old extractor returns an empty string for the real AISOS-1981 raw Jira ADF payload because its top-level description node is bulletList
• verified the fixed JiraClient.get_issue('AISOS-1981') path renders non-empty ORIGINAL CONTENT in the Epic revision prompt
• uv run pytest tests/unit/integrations/jira/test_client.py
• uv run ruff check src/forge/integrations/jira/models.py tests/unit/integrations/jira/test_client.py
• uv run pytest tests/unit/integrations/jira tests/unit/workflow/nodes/test_epic_decomposition.py tests/unit/workflow/nodes/test_task_generation.py tests/unit/workflow/nodes/test_trace_context_enrichment.py tests/unit/orchestrator/nodes/test_generate_prd.py tests/unit/api/routes/test_jira_webhook.py