[Chat] Fix MessageNormalizer round-trip for AssistantMessage with tool calls

[Amoifr]

Q	A
Bug fix?	yes
New feature?	no
Docs?	no
Issues	Fix #1972
License	MIT

Problem

When a conversation containing an AssistantMessage (or ToolCallMessage) with tool calls is persisted through MessageNormalizer and later restored, denormalize() crashes on Undefined array key "function".

Root cause

• MessageNormalizer::denormalize() reads the OpenAI-style wrapped shape: {id, function: {name, arguments: JSON string}}.
• MessageNormalizer::normalize() delegates the tool call payload to the outer Serializer. When ToolCallNormalizer is in the chain, the wrapped shape is produced; otherwise the default ObjectNormalizer falls back to the flat shape {id, name, arguments: array}.
• ToolCallNormalizer was never registered as a service in ai-bundle. In real Symfony apps, the outer serializer therefore produced the flat shape, and denormalize crashed. The chat component's existing round-trip tests only passed because they wire ToolCallNormalizer manually.

Fix (two complementary changes)

1. ai-bundle — register Symfony\AI\Platform\Contract\Normalizer\Result\ToolCallNormalizer with the serializer.normalizer tag, so the outer serializer produces the wrapped shape the denormalizer expects.
2. chat — make MessageNormalizer::denormalize() tolerate both shapes (wrapped and flat). This lets apps restore conversations that were persisted before this fix landed (the reporter's own DB rows are in the flat form).

Test plan

• src/chat — 41 tests pass, including 2 new regression tests exercising the flat-shape path without ToolCallNormalizer in the chain (testItCanDenormalizeAssistantMessageWithFlatToolCalls, testItCanDenormalizeToolCallMessageWithFlatToolCall).
• src/ai-bundle — 239 tests pass (+1 from this PR verifying the new service registration and tag), identical error count to main (8 pre-existing errors tied to the optional symfony/ai-ovh-platform package, unrelated to this change).
• phpstan clean on src/chat.

Notes

• Thanks to @sjahns for the precise diagnosis in the issue — the wrapped-vs-flat asymmetry was described clearly enough that the fix direction was obvious.
• The toolCallFromPayload() helper centralizes the tolerance so there's a single place to revisit if we ever want to drop the flat-shape fallback later.