When `Client.clone()` was called with headers containing a `User-Agent`,
it would overwrite the base client's `User-Agent` rather than merging.
This loses the SDK's default UA token when callers supply their own.

## Changes

- **`packages/common/src/microsoft_teams/common/http/client.py`**
- Added `_merge_headers(base, overrides)` helper: performs a
case-insensitive `User-Agent` merge (space-concatenated, token-based
dedup to avoid substring false positives); all other headers follow
standard last-write-wins semantics
- `Client.clone()` now calls `_merge_headers()` instead of a plain dict
unpack

- **`packages/common/tests/test_client.py`**
- Added 7 tests covering: preservation without overrides, merge when
both sides define UA, non-UA overrides leave UA untouched,
case-insensitive key matching, duplicate-token suppression, substring
false-positive guard, and override-only UA when base has none

## Example

```python
client = Client(ClientOptions(headers={"User-Agent": "teams-py/1.0"}))
clone = client.clone(ClientOptions(headers={"User-Agent": "myapp/2.0"}))

# Before: "myapp/2.0"  (SDK UA lost)
# After:  "teams-py/1.0 myapp/2.0"
print(clone._options.headers["User-Agent"])
```

> [!WARNING]
>
> <details>
> <summary>Firewall rules blocked me from connecting to one or more
addresses (expand for details)</summary>
>
> #### I tried to connect to the following addresses, but was blocked by
firewall rules:
>
> - `releases.astral.sh`
> - Triggering command: `/home/REDACTED/.local/bin/uv uv sync
--all-packages --group dev` (dns block)
>
> If you need me to access, download, or install something from one of
these locations, you can either:
>
> - Configure [Actions setup
steps](https://gh.io/copilot/actions-setup-steps) to set up my
environment, which run before the firewall is enabled
> - Add the appropriate URLs or hosts to the custom allowlist in this
repository's [Copilot coding agent
settings](https://github.com/microsoft/teams.py/settings/copilot/coding_agent)
(admins only)
>
> </details>

<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>

> Implement the change described by PR microsoft/teams.ts#508 ("Merge
User-Agent headers when cloning HTTP client") in the Python SDK
repository @microsoft/teams.py.
> 
> Goal:
> - When the HTTP client is cloned (or a new client is created from an
existing client/session), ensure that `User-Agent` headers are merged
rather than overwritten.
> - Preserve any existing default `User-Agent` set by the library while
also incorporating a user-provided `User-Agent` (or additional UA
tokens) when cloning.
> 
> Requirements:
> 1. Locate the HTTP client implementation and any clone/copy method(s)
in the repository.
> 2. Identify how headers are stored (e.g., requests.Session headers,
httpx.Client headers, custom dict) and how cloning currently works.
> 3. Modify cloning logic so that:
> - If both source and override headers contain `User-Agent`, the
resulting `User-Agent` should include both (e.g., concatenated with a
space, or comma per convention) without duplicating identical values.
>    - If only one side has `User-Agent`, keep it.
>    - Header merging should be case-insensitive for the header key.
> 4. Add/adjust unit tests to cover:
>    - Cloning without overrides preserves original `User-Agent`.
> - Cloning with headers override that includes `User-Agent` results in
merged value.
> - Cloning with headers override without `User-Agent` leaves original
UA intact.
>    - Case-insensitive behavior (`user-agent`, `User-Agent`).
> 5. Update any relevant docs or changelog entry if the repo uses one.
> 
> Reference:
> - PR: microsoft/teams.ts#508
> 
> Notes:
> - Ensure behavior is consistent across sync/async HTTP clients if both
exist.
> - Keep backward compatibility with existing public APIs.
> 
> 
> The following is the prior conversation context from the user's chat
exploration (may be truncated):
> 
> User: Can you implement this in @microsoft/teams.py ?
> 
> Assistant: I’ve queued up the implementation for `microsoft/teams.py`
and I’m now waiting on your confirmation in the prompt to proceed with
making the code changes and opening the PR.
> 
> 


</details>



<!-- START COPILOT CODING AGENT SUFFIX -->

*This pull request was created from Copilot chat.*
>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: heyitsaamir <[email protected]>
Co-authored-by: heyitsaamir <[email protected]>
Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>

## Summary
- Merges all changes from `main` since v2.0.7 into `release`
- Sets `version.json` to stable `2.0.8`

### What's included
- **feat:** Sovereign cloud support (GCCH, DoD, China) (#500)
- **feat:** Add missing endpoints — Paged Members, Meeting Notifications
& client gaps (#516)
- **feat:** Graceful stream cancellation on 403 (#513)
- **feat:** GitHub issue analysis → Teams notification workflow (#517)
- **fix:** Improve error message when app credentials are missing (#527)
- **fix:** Surface Graph API error body in GraphError (#524)
- **fix:** Resolve missing deps, broken JSON, type errors, and typos in
CLI templates (#521)
- **fix:** Drain entire queue per flush cycle (#520)
- **fix:** Merge User-Agent headers when cloning HTTP client (#508)
- Dependency bumps: hono, axios, vite, @hono/node-server

## Post-merge
1. Trigger the [release
pipeline](https://dev.azure.com/DomoreexpGithub/Github_Pipelines/_build?definitionId=52&_a=summary)
for `release` with **Public** publish type
2. Bump `version.json` on `main` to `2.0.9-preview.{height}`

🤖 Generated with [Claude Code](https://claude.com/claude-code)