refactor(parameters): widen API return types from dict[str, int] to dict[str, Any]

[MAfarrag]

Context

PR #149 added `| None` to Figshare API return types but kept the
overly narrow `dict[str, int]` value type. The type-design-analyzer
and code-reviewer both flagged this.

Problem / Current Behaviour

Figshare API responses contain strings, lists, nested dicts, booleans,
and integers. The current `dict[str, int]` annotation is misleading
and causes downstream type issues (e.g., `list_article_versions`
needs `# type: ignore[return-value]`).

Additionally, `ParameterManager.get_parameter_set_details` can return
`None` but some callers (e.g., `download_files`) don't guard against
it.

Affected locations

File	Symbol	Notes
`src/Hapi/parameters/parameters.py:59`	`send_request`	Returns `dict[str, int] | None`
`src/Hapi/parameters/parameters.py:92`	`get_article_version`	Same
`src/Hapi/parameters/parameters.py:116`	`list_article_versions`	Returns `list[dict]`
`src/Hapi/parameters/parameters.py:297`	`get_parameter_set_details`	Returns `dict | None`
`src/Hapi/parameters/parameters.py:351`	`download_files`	Calls details without None guard

Proposed Solution

1. Change return types to `dict[str, Any] | None`
2. Add None guards in `download_files` and other callers
3. Remove `# type: ignore[return-value]` from `list_article_versions`

Effort Estimate

Size: `S`

Definition of Done

• All API return types use `dict[str, Any]` instead of `dict[str, int]`
• All callers guard against `None` returns
• No `# type: ignore[return-value]` needed
• mypy passes

[MAfarrag]

Closed: created in error. These are follow-up items from PR #152 review, not standalone issues.