Allow configuring FastMCP transport setting in the same way as other configuration

[jvdmr]

`run_async()` hardcoded `"stdio"` as the fallback when no transport was specified, which meant users running third-party servers (like AWS MCP servers in ECS containers) had no way to change the default transport without modifying source code. The CLI and `fastmcp.json` provided workarounds, but a bare `mcp.run()` call was locked to stdio.

This adds a `transport` field to `Settings` so that `FASTMCP_TRANSPORT=http` (or any valid transport) controls the default. `run_async` reads from the setting instead of hardcoding `"stdio"`, while explicit `transport=` arguments and CLI `--transport` flags still override it. Fits naturally alongside the existing `FASTMCP_HOST` and `FASTMCP_PORT` settings.

```python

Third-party server code you can't modify:

mcp.run() # reads FASTMCP_TRANSPORT from env

Or in your own code:

import fastmcp
fastmcp.settings.transport = "http"
```

Closes #1795

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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Walkthrough

The transport configuration option has been made configurable through the Settings class. A new transport field was added to Settings with type Literal["stdio", "http", "sse", "streamable-http"] and a default value of "stdio". The transport selection logic in the transport mixin was updated to delegate to this configuration field when no transport is explicitly provided, rather than using a hardcoded default value.

🚥 Pre-merge checks | ✅ 6

✅ Passed checks (6 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and specifically describes the main change: making transport configurable via settings mechanism, which is the primary objective of the PR.
Description check	✅ Passed	The PR description clearly explains the problem, solution, and provides code examples. It includes the closing issue reference but lacks coverage of testing and documentation updates.
Linked Issues check	✅ Passed	The PR fully implements the requirement from issue #1795: adding a transport field to Settings that can be configured via FASTMCP_TRANSPORT environment variable, allowing users to override the hardcoded stdio default.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to the linked issue: adding transport field to Settings and updating run_async() to use it instead of hardcoding stdio. No extraneous modifications detected.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Merge Conflict Detection	✅ Passed	✅ No merge conflicts detected when merging into main

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[jlowin]

@jvdmr now that #3148 is in, we can get this merged - apologies for the significant delay. This will go out after 3.0 releases.