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

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[cornelcroi]

Hey @nuthalapativarun, a few things need to be fixed before this can merge.

There is a bug in save_chat_message — when a consecutive-role duplicate is detected, the method returns the raw result from _fetch_raw which still has placeholder strings in it instead of the original values. The caller gets masked data back. Also, fetch_all_chats applies _reverse_content_filters differently from the other fetch methods — it has an isinstance guard that silently skips the reversal when content is not a list, so placeholders can leak through. Both of these need tests too. And the docs page for DynamoDB storage needs to be updated to document the new content_filters parameter. CI also hasn't run on the branch.

One thing worth calling out in the PR description: the filter replacement works by doing a json.dumps then naive string replace, so if a filter string contains JSON special characters like quotes or backslashes it can produce invalid JSON. Worth documenting as a known limitation. Also interesting to note — the Swift equivalent (TransformingChatStorage) deliberately does not restore the original values on read, while this PR does. Both approaches are valid but they are different behaviors and worth mentioning.