Add type-safe Jira issue models with fluent builders

[yechielb2000]

Summary

• Introduces a new atlassian.models.jira package that replaces manual JSON/dictionary construction with typed dataclasses, fluent builders, and a centralized serializer for all Jira issue operations.
• Covers the full lifecycle: create (IssueBuilder + serialize), update (UpdateBuilder), transition (TransitionBuilder), comment (Comment model), and deserialize (from_dict on all value objects).
• Includes 86 pytest tests, a py.typed PEP 561 marker, and a convenience import alias (atlassian.jira_models).

New modules

Module	Purpose
fields.py	Frozen value-object dataclasses (Project, Priority, User, etc.) with to_dict()/from_dict()
issues.py	JiraIssue base + Task/Bug/Story/Epic/SubTask with __init_subclass__ auto-registry
builders.py	Generic IssueBuilder[T] with per-type builders, ADF bridge, .validate() chaining
serializer.py	FieldMapping + serialize()/to_fields_dict()/bulk_serialize()
adf.py	ADFBuilder for Atlassian Document Format rich-text descriptions
validation.py	Pre-flight validate()/validate_or_raise()
update.py	UpdateBuilder for issue_update with set/add/remove operations
transition.py	Transition model for set_issue_status()
comment.py	Comment + Visibility for issue_add_comment()

Quick usage example

from atlassian.models.jira import bug, serialize

payload = (
    bug()
    .project("PROJ")
    .summary("Login button broken on Safari")
    .priority("High")
    .labels("safari", "regression")
    .description(lambda d: d.paragraph("Steps to reproduce..."))
    .build_payload()
)

jira.create_issue(fields=payload["fields"])

Test plan

• All 86 unit tests pass (pytest tests/test_jira_models.py -v)
• Manual review of public API surface and docstrings
• Verify import paths work from consumer code