fix(messages): add InstructionPart to ModelRequestPart discriminated union

[devteamaegis]

Problem

InstructionPart has part_kind: Literal['instruction'] = 'instruction' and its
docstring states "Part type identifier, used as a discriminator for deserialization."
It can appear in ModelRequest.parts during normal agent runs, but it was not
included in the ModelRequestPart discriminated union.

As a result, any serialized message history containing an InstructionPart in its
parts list would fail to reload:

from pydantic_ai import InstructionPart, ModelMessagesTypeAdapter, ModelRequest, UserPromptPart

req = ModelRequest(parts=[
    UserPromptPart(content='Hello'),
    InstructionPart(content='Be concise.'),
])

serialized = ModelMessagesTypeAdapter.dump_json([req])  # succeeds with warnings
ModelMessagesTypeAdapter.validate_json(serialized)       # raises ValidationError!
# Input tag 'instruction' found using _model_request_part_discriminator()
# does not match any of the expected tags:
# 'system-prompt', 'user-prompt', 'tool-search-return', 'tool-return', 'retry-prompt'

This silently corrupted message histories in Temporal/durable-exec workflows, resumed
runs, and any code that persists history to JSON and reloads it.

Fix

Add Annotated[InstructionPart, pydantic.Tag('instruction')] to the ModelRequestPart
union (one line change in messages.py).

Tests

Added TestInstructionParts::test_instruction_part_in_parts_roundtrip — a regression
test that round-trips a ModelRequest containing both static and dynamic
InstructionPart instances alongside a UserPromptPart through
dump_json → validate_json and asserts full fidelity.

Fixes #5696.