RavenDB-24609 Add GetConversationMessages operation for AI agent conversations

[redknightlois]

Issue

https://issues.hibernatingrhinos.com/issue/RavenDB-24609

Source (C# client): ravendb/ravendb#22619

What changed

Brings the GetConversationMessages read operation to the Python client, mirroring the C# client
(ravendb/ravendb#22619). It fetches an AI agent
conversation's message history from
GET /databases/{db}/ai/agent/conversation/messages, with:

• Timestamp paging — before and after cursors (mutually exclusive), serialized in Raven's
  default UTC format (yyyy-MM-ddTHH:mm:ss.fffffffZ), UTC-coerced before formatting so a naive or
  non-UTC datetime yields a correct cursor.
• Page size — max messages to return (default: unbounded).
• Detail level — Simple (default), Detailed, Full; passed to the server for filtering.

New module ravendb/documents/operations/ai/agents/get_conversation_messages_operation.py:

• GetConversationMessagesOperation / GetConversationMessagesCommand — a MaintenanceOperation
  with an inner read RavenCommand.
• GetConversationMessagesOptions — parameter object with eager client-side validation (rejects an
  empty/whitespace conversation_id, before and after together, and page_size <= 0).
• DTOs AiConversationMessagesResult, AiConversationMessage, AiToolCallResult — snake_case Python
  attributes with from_json/to_json mapping to/from the server's PascalCase keys.
• Enums AiMessageRole (System, User, Assistant, Summary, Internal) and
  AiConversationDetailLevel (Simple, Detailed, Full).
• AiUsage is reused from run_conversation_operation — not duplicated.

A convenience AiOperations.get_conversation_messages() accepts either a conversation-id string or a
GetConversationMessagesOptions.

Response-timestamp parsing normalizes the fractional-seconds precision so it is compatible with the
package's declared Python floor (~=3.9): datetime.fromisoformat before 3.11 accepts only 3 or 6
fractional digits and rejects a Z suffix, whereas Raven emits 7-digit (100 ns tick) ...Z
timestamps — both are normalized before parsing.

Tests: 43 unit tests (no mocks, no live server), covering URL/query construction, validation,
PascalCase↔snake_case round-trips through the real parser with representative server JSON (nested
tool calls, usage, sub-conversation ids), null-response handling, and datetime formatting/parsing.

Files (5): the new operation module and its test file; re-exports in
ravendb/documents/operations/ai/agents/__init__.py and ravendb/__init__.py; the convenience
method in ravendb/documents/ai/ai_operations.py.

Checklist

• Tests added or existing tests cover the change
• Breaking change (explain above if checked)

Known risks / not verified here

• Python 3.9 floor: the datetime-precision fix is verified by construction (the normalized
  string falls within the fractional-digit/offset grammar that pre-3.11 fromisoformat accepts) and
  by the suite passing on 3.13/3.14, but was not executed under a <3.11 interpreter — none is
  installed here. Worth a CI run on 3.9 to confirm.

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_{Implemented by hugin-bot, created on behalf of @redknightlois.}