Recipient headers for specific text searches

[u7327620]

What does this PR do, and why?

Previously, when text searching, messages appear adjacent under the same heading, this can be confusing since they appear to be sequential but there may be messages in-between that don't meet the search criteria. PR adds logic to add recipient headers if the search isn't a specific dm or a topic narrow

Outstanding aspect(s)

• Uncertain if automated tests are covering the feature or not (new to pytest)
• Related follow-ups aren't fixed in this PR ... should they be?

External discussion & connections

• Discussed in #zulip-terminal in topic
• Fully fixes Search results should show recipient headers for every message #1550
• Partially fixes issue #
• Builds upon previous unmerged work in PR #
• Is a follow-up to work in PR #
• Requires merge of PR #
• Merge will enable work on #

How did you test this?

• Manually - Behavioral changes
• Manually - Visual changes
• Adapting existing automated tests
• Adding automated tests for new behavior (or missing tests)
• Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• It is a minimal coherent idea
• It has a commit summary following the documented style (title & body)
• It has a commit summary describing the motivation and reasoning for the change
• It individually passes linting and tests
• It contains test additions for any new behavior
• It flows clearly from a previous branch commit, and/or prepares for the next commit

Visual changes

Before:
After: