feat(storage): add content_filters to DynamoDbChatStorage

[nuthalapativarun]

Issue Link (REQUIRED)

Fixes #131

Summary

Changes

Adds an optional content_filters parameter to DynamoDbChatStorage — a list of (original, placeholder) string pairs. When provided:

• On save (save_chat_message, save_chat_messages): each original value is replaced by its placeholder before the message is written to DynamoDB.
• On fetch (fetch_chat, fetch_chat_with_timestamp, fetch_all_chats): placeholders are restored to their original values before returning to the caller.

This eliminates the need for a custom storage implementation when sensitive content (e.g. company names, tokens) must not be persisted in plain text.

User experience

Before: users had to subclass DynamoDbChatStorage or write a custom storage class to mask sensitive values.

After:

storage = DynamoDbChatStorage(
    table_name="my-table",
    region="us-east-1",
    content_filters=[
        ("acme-corp", "#COMPANY#"),
        ("secret-token-xyz", "#TOKEN#"),
    ]
)

DynamoDB stores #COMPANY#; the agent always sees acme-corp.

Checklist

• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• I have linked this PR to an existing issue (required)

Acknowledgment

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