ce user agent possible refactor

(I'm Claude, posting on Patrick's behalf.)

Not needed for this PR, but a possible future refactor: this PR's `_ce_user_agent()` overlaps with the hand-rolled UA strings in `bin/lib/rust_crates.py`, which already build the same `product (comment)` shape for crates.io:

- `get_builder_user_agent_id()` -> `"Compiler Explorer Library Builder (github.com/compiler-explorer/infra)"`
- `get_manual_user_agent_id()` -> `"Compiler Explorer (github.com/compiler-explorer/infra)"`

Both places construct `product (comment)`; the only real difference is the comment (this PR uses `Compiler Explorer`, Rust uses the repo URL, which is what crates.io's crawler policy actually asks for -- a contact URL).

Since `installation_context.py` is a heavyweight module and `rust_crates.py` is intentionally tiny (just `json`/`urllib`), the clean combine would be a small shared `bin/lib/user_agent.py` that both import, rather than having `rust_crates` pull in `installation_context`:

```python
from __future__ import annotations

import requests

CE_USER_AGENT_URL = "https://github.com/compiler-explorer/infra"

def ce_user_agent(product: str = "") -> str:
"""Build a CE User-Agent as `product (+url)`."""
product = product.strip() or f"python-requests/{requests.__version__}"
return f"{product} (+{CE_USER_AGENT_URL})"
```

Then `installation_context` drops its local helper and calls `ce_user_agent(agent)`, and `rust_crates` becomes `ce_user_agent("Compiler Explorer Library Builder")` / `ce_user_agent("Compiler Explorer")`. Net effect: the fetch default keeps its `python-requests/{version}` prefix (preserving today's behavior) while every CE request also carries the contact URL.

Again, no need to do this here -- just flagging it so we don't grow a third UA convention later.

_Originally posted by @partouf in https://github.com/compiler-explorer/infra/issues/2237#issuecomment-5013345528_

Read the `_ce_user_agent()` helper in `installation_context.py` and `get_builder_user_agent_id()` / `get_manual_user_agent_id()` in `bin/lib/rust_crates.py`, then compare their current imports and User-Agent behavior. The refactor is done when both use the shared `bin/lib/user_agent.py` helper, the fetch default preserves its `python-requests/{version}` prefix, and CE requests include the contact URL.

Written by the indexing model from the issue text.