make clearer that a str is truncated

[Clement-Lelievre]

Description

@alexmojaki
related to #476

Currently, an image appears as base 64 in LogFire, except it usually gets truncated and it is not clear enough IMO, because:

• the b64 str does not indicate it (by saying smthg like [TRUNCATED] or ending with ... )
• it's ofc a valid str, ending with a quote
• but its hard limit is very high, so it can get really big and look like an actual, full b64 str => on opening, will result in issues like b64 padding errors

One has to check the Details tab in LogFire and look for a truncation info

So my request is: can you pls indicate explicitly when a str is truncated, inside the str itself, at a place where it's easily seen (so not at the end of a very long str)?

Python, Logfire & OS Versions, related packages (not required)

logfire="4.14.2"
platform="macOS-15.1.1-arm64-arm-64bit-Mach-O"
python="3.13.2 (main, Feb  4 2025, 14:51:09) [Clang 16.0.0 (clang-1600.0.26.6)]"
[related_packages]
requests="2.32.4"
pydantic="2.11.7"
fastapi="0.121.1"
openai="2.2.0"
protobuf="5.29.5"
rich="14.0.0"
executing="2.2.1"
opentelemetry-api="1.38.0"
opentelemetry-exporter-otlp-proto-common="1.38.0"
opentelemetry-exporter-otlp-proto-http="1.38.0"
opentelemetry-instrumentation="0.59b0"
opentelemetry-instrumentation-asgi="0.59b0"
opentelemetry-instrumentation-fastapi="0.59b0"
opentelemetry-instrumentation-google-genai="0.4b0"
opentelemetry-instrumentation-httpx="0.59b0"
opentelemetry-proto="1.38.0"
opentelemetry-sdk="1.38.0"
opentelemetry-semantic-conventions="0.59b0"
opentelemetry-util-genai="0.2b0"
opentelemetry-util-http="0.59b0"