[BugFix][TVMScript] Fix invalid f-string format spec causing TypeError on Python 3.14 (#19362)

## Problem

On Python 3.14, any use of TVMScript raises a `TypeError` before the
module body is even parsed:

```
TypeError: unsupported format string passed to type.__format__
```

The traceback points to
`python/tvm/script/parser/core/diagnostics.py:120`:

```python
raise TypeError(f"Source for {obj:!r} not found")
```

## Root Cause

`{obj:!r}` is an invalid f-string expression. The `:` introduces a
`format_spec`, so `!r` is passed to `type.__format__` as a format string
— which it does not support.

The intended syntax for a `repr()` conversion is `{obj!r}` (no colon).

Python 3.14 re-implemented f-string parsing under [PEP
701](https://peps.python.org/pep-0701/) and now strictly validates
format specs, surfacing this latent bug. Python 3.10–3.13 silently
passed the invalid spec to `__format__` and happened not to raise in
most code paths, so the bug went unnoticed.

## Fix

```diff
- raise TypeError(f"Source for {obj:!r} not found")
+ raise TypeError(f"Source for {obj!r} not found")
```

One character change. Valid across all Python versions >= 3.6.

## Testing

Verified on Python 3.14.2 (darwin/arm64):

- TVMScript `ir_module` + `prim_func` parses and compiles correctly
after the fix
- Full TVMScript test suite: **628 passed, 1 xfailed** (the 1 failure in
`test_tvmscript_roundtrip.py::test_roundtrip[relax_symbolic_size_var]`
is pre-existing and unrelated to this change)